Vector Expressions
Pass flatten=True for all par_loop Arguments

The coefficients in the form were missed in the great flatten change.
Add test solving identity problem on VectorFunctionSpace
Add (failing) parallel test of vector function space assembly
Remove invalid expression tests

UFL doesn't allow one to apply scalar operators (such as ln) to
VectorFunctionSpaces, so now we have the correct ufl_element, these are
no longer expected to work.
Fix building VectorFunctionSpaces on extruded meshes

We need to build an OuterProductVectorElement not a VectorElement in
this case.
Report error if assembling vector functions in parallel

These require more work, so present a reasonable error message rather
than silently producing wrong results.
Support writing vector fields to VTK files