Separate nullspace and nearnullspace, add transposenullspace

[cmdoug]

Attempt to fix #371 reported by @echarala.

[prj-]

LGTM, @echarala, could you please confirm this fixes your issue?

[echarala]

Yes, it should. I will try it in my problem. Nice fix, guys!

[cmdoug]

@prj- note that the CI pipeline didn’t auto-run for this PR yet. It passes make check on my end, though.

[prj-]

Yeah, there is no reason to have it fail make check, since the code (or the combination of both null spaces) is largely untested. It would be nice to have a minimalist example that combines both and that was previously failing, but I'm guessing it may be a little bit difficult to extract from your code, @echarala? BTW, I'm waiting on your confirmation to get this integrated, so please let us know.

[cmdoug]

While this is in the works, should I also add an interface to TransposeNullSpace? Also, it looks like PETSc is missing a ConjugateTransposeNullSpace implementation. This would be nice to have for completeness. I could see both being useful for bifurcation problems with iterative solvers.
EDIT: just added transposenullspace. Let me know if you'd prefer that this were wrapped into a separate PR. I think some work is needed on the PETSc side before we can add a conjugatetransposenullspace parameter.

[prj-]

Would you be OK with me merging this, @cmdoug?

[cmdoug]

Sure thing. Will do some work on the PETSc side next couple weeks and start a new one for the remaining stuff.

[prj-]

Sounds good, thanks!