Make MassOperator's diagonals true diagonals, not lumped ones.

Make MassOperator's diagonals true diagonals, not lumped ones.

The previous implementation used row sums (i.e., mass lumping) instead of
computing the diagonal. This should be changed for two reasons:
1. It's not really the diagonal (and its inconsistent with LaplaceOperator).
2. Lumped mass matrices are only positive for either low-order elements (where
   the basis functions are all positive) or very well-behaved elements (like
   FE_Q). In particular, the values on the diagonal are either zero or negative
   for TET10 if we use lumping, which isn't going to work.