renamed polyder test suite
added polyint, fixed ndgridn for dimension > 2

The kron() function is rather useful in constructing stuctures
arrays, and we use it in ndgridn used for polyder and polyint
methods.  Matlab's kron can only be used with 2 arguments, while
Octave's takes arbitrarily many -- we add a kronn() subfunction
to ndgridn.m for the benefit of the weaker software.