Implement split for nested mixed elements

Modified example from @pfarrell

V = FunctionSpace(mesh, "CG", 1)
A = V*V
B = A*V

b = Function(B)
a, c = split(b)
(a0, a1) = split(a) # this fails and I'd like it to work

Fixing this should be just a little mental exercise, the implementation of a better split won't affect anything else.

I also think split(vector_value_element_function) should return the components, treating it as if it was a VectorElement.

A related but separate issue is how to make dolfin f.split() work nicely with ufl.split(f) somehow.

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