BUG: Fixed issue #5156: segfault on read_csv
Merge pull request #5268 from guyrt/issue-5156-segfault

BUG: Fixed issue #5156: segfault on read_csv
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
EHN: DataFrame can now sort a single column or label by all three parameters:
     kind, ascending and na_position.

Added _nargsort, and extension of np.argsort that has ascending and na_position
parameters.
Rewrite _nargsort
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.

EHN: Add function `array_equivalent` to test if arrays are equal except
for NaN elements at corresponding locations.