Add cutplane command -- pseudocolored plane on principal axis. Can be colored
by scalar or vector. Rename 'volumeslice' -> 'slice' in protocol. Fix PseudoColor? for case when dataset has only cell field vectors. Add variable
to Makefile for HAVE_GLYPH3D_MAPPER. Should really be a configure test for
VTK >= 5.8. Remove VTK legacy method support cruft from custom 2D axes class.
