Add Numpy-like functions

[bleyerj]

I found it sometimes painful to use UFL when reshaping matrix/vectors, etc.

It would be useful to have UFL functions mimicking numpy like behavior such as:
flatten, reshape, vstack, hstack, concatenate, etc.

I would be happy to make a proposal if we agree on this

[dham]

Presumably all of these are syntactic sugar for combinations of indexing on the one hand, and as_vector and as_tensor on the other. That being the case, this seems fairly unobjectionable.