Tour Interp Menu

This menu gives a choice of interpolation type for the tour.  The
default is geodesic interpolation, which interpolates between
planes along the shortest path.  The two additional types,
Householder and Givens interpolate between specific bases in the
planes.

For more information see Buja, A., Asimov, D., Hurley, C. (1989)
"Methods for Subspace Interpolation in Dynamic Graphics", Bellcore
Technical Memorandum. This has no information on the Householder and
Givens methods, but a review including these is underway and 
may be requested from dicook@iastate.edu.