[WIP] MultiFabRegister: throw in get

[ax3l]

Close #5319
Follow-up to #5230

• Throw a runtime exception instead of returning a nullptr if a field is requested via the getter.
• update logic to ensure this passes all tests

[EZoni]

Discussed with @RemiLehe offline:

The functions get_mr_levels and get_mr_levels_alldirs fails by definition when we call it on coarse-patch MultiFabs, e.g., current_cp, because those are allocated only on levels greater than level 0 while the function attempts to return all levels starting from 0.

We will try to add to get_mr_levels and get_mr_levels_alldirs a function argument, e.g., skip_level_0, such that the function returns a nullptr for level 0 instead of calling the internal functions of the MultiFabRegister and throwing an exception.