doc fixes, rather serious error in new interp_matrix() help text fixed
more appropriate grid scaling for paramCylinder in extreme cases
fixed annoying bug in linesegments

so split my too-clever 2d/3d code into two functions
fixes for cpf for semicircle and arc

default center to (0,0), swap bdy label for semicircle to match arc (arbitrary anyway)
cpLine sanity check to avoid switching dir and pt, + unit test
cpRotate: rotation of a given cpfun
cpfun and paramfun should have defaults (or so my unittests tell me)