index into multi-index past the lexsort depth
Merge branch 'mi-key-loc' of https://github.com/behzadnouri/pandas into behzadnouri-mi-key-loc
TST: provide a clear argument to assert_produces_warning to guarantee that we are clearing
     specific warnings before trying to catch them (in case they have already happened)
     requires the user to specificy the exact class where they are coming