FInAT needs a FiniteElement factory function

[connorjward]

From @dham in FInAT/FInAT#109

In order to move finite elements from UFL into FInAT, we need a function which returns finite elements given cell, family, degree, and potentially variant.

This also requires encoding all that information into FIAT or FInAT elements.

[pbrubeck]

Could this be solved by #120? We moved tsfc.finatinterface.py into the FIAT repo as finat.element_factory.py. The function create_element converts UFL elements into (cached) finat elements.

[pbrubeck]

Nevertheless, FInAT elements do not know about the UFL parameters that were used to construct them.

[connorjward]

I think what we really want is to unify FInAT and UFL elements into a single class that inherits from ufl.AbstractFiniteElement.

I guess your PR does achieve the factory functionality described here but we should be taking this further (in future).