ENH: support for removing unused levels of a MultiIndex (interally)#15700

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jreback wants to merge 3 commits intopandas-dev:masterfrom

Commits on Mar 22, 2017

BUG: construct MultiIndex identically from levels/labels when concatting
jreback
committed
incorrectly raising KeyError rather than UnsortedIndexError, caught by doc-example
jreback
committed
support for removing unused levels (internally)
jreback
committed