TODO 

cif++
- validate range
- validate child-parent relationships (is parent available? e.g.)

- improve API

PDB2CIF
- implement remark 3 for all known programs
- implement other remarks 
- implement hetero field in pdb2cif

- DNA one letter code generation (X?)

- hetero field in entity_poly_seq, pdbx_poly_seq_scheme

- chem_comp.type? chem_comp genereren uit db?
	- partial charge calculation
	- water for peptides?

cif-grep
- more options for cif-grep
