EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.
Merge pull request #5231 from unutbu/nan-sort

EHN/FIX: Add na_last parameter to DataFrame.sort. Fixes GH3917
BUG: Bug in consistency of groupby aggregation when passing a custom function (GH6715)