Raise error if trying to assemble on MFS + VFS

Assembling forms on MixedFunctionSpaces composed in part of a
VectorFunctionSpace is currently unimplemented so raise a useful error
message instead of going ahead and trying to build a kernel that does
the wrong thing.
Add coding cooking to Nitsche bcs test
Merge pull request #93 from firedrakeproject/fix/mfs-vfs-error

Raise a useful error message when trying to assemble on mixed space that
contains a VFS.