grib_api: Grib API keys

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# Grib API keys

The GRIBEX routine used at ECMWF to encode and decode GRIB messages works on a number based table to retrive all the information from the message. This approach forces the user either to learn a code table or to use the documentation intensively. With grib_api a key name based access is provided so that all the information contained in the GRIB message is retrieved through alphanumeric names.
All the key names are built from the official WMO documentation on the GRIB edition 1 and 2 coding standard removing the spaces in the key description and capitalizing the initials so that the caption:

    identification of originating generating centre

is transformed into the key name

    identificationOfOriginatingGeneratingCentre

Some short names (aliases) are also provided, e.g. "centre" is an alias for identificationOfOriginatingGeneratingCentre. The names are always easily releated to the meaning of their value.
A different set of keys is available for each message because the content is different. It is easy to find the keys available in a message by using the GRIB tools (grib_dump) or the library (keys_iterator.c).

## Coded and Computed keys

There are two different types of keys: coded and computed.
The coded keys are directly linked to octets of the GRIB message and their value is obtained by only decoding the octets. A list of all the coded keys in a message can be obtained using grib_dump without any option (use the -a option to obtain also their aliases).
The computed keys are obtained by combining other keys (coded or computed) and when their value is set all the related keys are set in a cascade process.
These keys provide a synthesis of the information contained in the GRIB message and are a safe way to set complex attributes such as the type of grid or the type of packing. They are also helpful in the interpretation of some octets such as the scanning mode whose bits are related to the way of scanning the grid. In this case the computed keys:

    iScansNegatively
    jScansPositively
    jPointsAreConsecutive
    alternativeRowScanning (available only for edition 2)

will provide access to single bits of the scanning mode octect hiding its structure from the user.
The keys can also have some attributes as read only, which means that the key cannot be set (e.g. 7777 at the end of the message), or edition specific that is the attribute of all the keys having different values in the two editions (e.g. longitudeOfFirstGridPoint) or being present in one edition only (e.g. alternativeRowScanning).
Moreover there are some computed keys that cannot be "get" and can be considered as functions acting on the grib in some way. These keys are always characterised by a predicate in their name (e.g. setDecimalPrecision).
For the computed keys we provide the following preliminary documentation that will be extended soon.

- MARS keywords.
  All MARS keywords are available. Some examples are:
  - date
  - param
  - levtype
  - levelist
  - step
  - stream
- angles in degrees.
  All the angle variables are provided in two versions, a native one with the units coded into the GRIB file and an edition independent one in degrees. It is always better to work with the "in degrees" version that is always provided through the key which has the same name of the native version with the suffix InDegrees

      longitudeOfFirstGridPoint -> longitudeOfFirstGridPointInDegrees
      latitudeOfFirstGridPoint -> latitudeOfFirstGridPointInDegrees
      longitudeOfLastGridPoint -> longitudeOfLastGridPointInDegrees
      latitudeOfFirstGridPoint -> latitudeOfLastGridPointInDegrees
      latitudeOfFirstGridPoint -> latitudeOfFirstGridPointInDegrees
      iDirectionIncrement -> iDirectionIncrementInDegrees
      jDirectionIncrement -> jDirectionIncrementInDegrees

- gridType
  The type of grid computed from the grid description section.
  - For both editions:
    - regular_ll
    - reduced_ll
    - mercator
    - lambert
    - polar_stereographic
    - UTM
    - simple_polyconic
    - albers
    - miller
    - rotated_ll
    - stretched_ll
    - stretched_rotated_ll
    - regular_gg
    - rotated_gg
    - stretched_gg
    - stretched_rotated_gg
    - reduced_gg
    - sh
    - rotated_sh
    - stretched_sh
    - stretched_rotated_sh
    - space_view
  - For edition 2 only:
    - triangular_grid
    - equatorial_azimuthal_equidistant
    - azimuth_range
    - cross_section
    - Hovmoller
    - time_section

- packingType
  The alghorithm used to pack data into the GRIB message.
  - For GRIB edition 1:
    - grid_simple
    - grid_simple_matrix
    - grid_simple_matrix_bitmap
    - grid_second_order
    - grid_second_order_different_width
    - spectral_complex
    - spectral_simple
    - grid_unknown
    - spectral_unknown
  - For GRIB edition 2:
    - grid_simple
    - grid_simple_matrix
    - grid_simple_matrix_bitmap
    - grid_complex
    - grid_complex_spatial_differencing
    - grid_jpeg
    - grid_png
    - grid_ieee
    - spectral_simple
    - spectral_complex
    - grid_simple_log_preprocessing

- setDecimalPrecision
  is a function key used to set the decimal precision see the grib_set page for usage.

- getNumberOfValues
  The number of values coded into the data section of the GRIB message