Raise error when DSP is used without IndexSet

[tjhei]

A very annoying and subtle bug is when you forget to provide an IndexSet to DynamicSparsityPattern: Everything works but the memory requirements for the data structure are O(total DoFs) instead of O(locally owned DoFs) in parallel.

My suggestion:
Add an AssertThrow() to distribute_sparsity_pattern() to check that the DSP has an IndexSet set, which is not the whole set.

Can you do this, @vyushut ?

[bangerth]

What do you do if you run a parallel code on one processor? What if you run it on two processors, but you only have one cell and consequently all DoFs on one process are locally owned?

[tjhei]

I would compare the IndexSet passed to distribute_sparsity_pattern() with the one stored inside the DSP: if the argument doesn't contain all DoFs, then the DSP should also not store all lines. I think all your cases should work (but we should test this as well!).

[tjhei]

It is a hard to catch bug as you can see in #16878, where none of the reviewers caught it (including myself). :-)

[bangerth]

Ha, yes, for sure. I think comparing the index sets is a good strategy!

[vyushut]

#17033
I subtract one IndexSets from another and expect the number of indexes to be zero.