Fix Hex20 shape functions; Add partition of unity tests.

Same as in the OGS-5 code in FEM/mathlib.cpp ( and in all previous versions of that code looked up to revision 6 in the svn, which is an import from 4.2.17.) Maybe the node ordering was different (for example (1, 1, 1) is the first point not the eighth).

The assumption that the current implementation is wrong is based on simple test for the first node of the hexahedron with coordinates (-1, -1, -1), where the shape function's value must be 1...