Don't use user_index. Use active_cell_index instead

[bangerth]

#425 introduces a number of uses of cell->user_index(), see the comments in the pull request that were not addressed before the merge. This is not necessary, we can use cell->active_cell_index() instead, but that function was only introduced in deal.II 8.3 whereas the automated tester uses 8.2.1.

We should revisit the issue after the next ASPECT release when we can decide that we want to require a newer deal.II release.