BUG/CLN: MI now checks level & label compatibility

Via verify_integrity kwarg. Internals all pass False.

CLN: Verify integrity by default on label/level setting
Merge pull request #5214 from jtratner/mi-validate-level-label-compat

BUG/CLN: MI now checks level & label compatibility
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917