Relax the restrictions on 64-bit PETSc.

[drwells]

This is from slack.

Since PetscInt is a signed integer, we should always

1. make a copy (no reinterpret_cast<PetscInt *>)
2. do a range check (since, for 32-bit indices, the maximum range is significantly different)

If we're doing both, then we don't need to restrict ourselves to matching DEAL_II_WITH_64BIT_INDICES and sizeof(PetscInt) == 8. That would simplify the CI set up a bit (and make things easier for users).

[bangerth]

Perhaps via a convert_to_PetscInt() function, rather than always doing it by hand in all places necessary?

[drwells]

That's a good idea