Add error checks for FE space ordering

[zatkins-work]

#1176 removes an error check in serac::FiniteElementVector which asserts that the DOF ordering is mfem::Ordering::byNODES. To prevent future regressions, we should do one (or both) of the following:

1. Add back this error check, instead checking that the ordering matches serac::ordering
2. Allow FiniteElementVector objects to have different orderings, but convert them to serac::ordering before using them. This is probably more complicated and error prone, but may be needed to support Tribol correctly.