Computes a pair-wise frequency table of the given columns. Also known as cross-tabulation.
cc mengxr rxin

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #5842 from brkyvz/df-cont and squashes the following commits:

a07c01e [Burak Yavuz] addressed comments v4.1
ae9e01d [Burak Yavuz] fix test
9106585 [Burak Yavuz] addressed comments v4.0
bced829 [Burak Yavuz] fix merge conflicts
a63ad00 [Burak Yavuz] addressed comments v3.0
a0cad97 [Burak Yavuz] addressed comments v3.0
6805df8 [Burak Yavuz] addressed comments and fixed test
939b7c4 [Burak Yavuz] lint python
7f098bc [Burak Yavuz] add crosstab pyTest
fd53b00 [Burak Yavuz] added python support for crosstab
27a5a81 [Burak Yavuz] implemented crosstab