MAINT: interpnd.pyx is not autogenerated
BUG: qhull: fix potential infinite loop in RidgeIter2D

For certain triangulations, RidgeIter2D could enter an infinite loop due
to degenerate simplices.  This commit replaces the RidgeIter2D by an
explicit list of neighboring vertices.

An explicit list is also more useful for higher-dimensional data.

Fixes gh-2359
Merge pull request #2697 from pv/fix-2359

BUG: qhull: fix potential infinite loop in RidgeIter2D