Rename and improve ProductFemSpace class

[yguclu]

1. The name ProductFemSpace is ambiguous: seems to be used only for multi-patch spaces, but a space of vector-valued fields (a VectorFemSpace) is also a product of FEM spaces...

   -> We should rename it as MultipatchFemSpace

2. The "product" ambiguity is made worse by the flag function Vh.is_product which means either multi-patch or vector-valued (see for instance in the FemField constructor where its is understood as vector-valued...)

   -> We should replace it by Vh.is_multipatch and Vh.is_vector_valued

3. The ProductFemSpace constructor may not return a ProductFemSpace, so the code is not easy to follow.

   -> We should use a factory function instead, with a clear docstring