Tour: grand tour and projection pursuit guided tour

This initiates a grand tour using the first three variables in the data
matrix.  The labels of the included variables are highlighted.  (If
there are fewer than three variables in the data, motion ceases.)

The grand tour method used here successively samples planes in p-space,
where p is the number of variables presently selected, and connects the
planes by moving along a geodesic interpolation path between them.
Motion is smooth between the starting and ending planes, but when the
ending plane is reached there is a slight delay while the next randomly
selected ending plane and consequent path are calculated.  By allowing
the grand tour to run uninterrupted the viewer can get a global view of
the linear combinations among the variables. As the tour runs, the
number of base planes passed is displayed in the box to the right of
the Backtrack button.

A variable may be added to or removed from the tour by clicking on the
corresponding variable selection box with the left or middle mouse
button.  There must be three or more variables included in the grand
tour, so it is not possible to remove a variable when only three are
active. The bars drawn inside the variable circles represent the
coefficients of the variables in the current plot.  The projection of a
bar on the horizontal axis of the variable circle corresponds to the X
coefficient of that variable, while the projection onto the vertical
axis of the variable circle corresponds to the Y coefficient.

This button can be activated from the keyboard by typing t or T.
