ENH/BUG Contingency table improvements

[kshedden]

• Add __str__ methods so that the table objects and results returned as bunches are informative when printed.

• Fix problems with make_df_square (it wasn't tested and was not working with current Pandas)

• Allow nested lists, e.g. [[3, 5], [2, 1]] to be passed to the Table2x2 initializer.

• Fix issues with the two from_data methods

• flake8 edits

[coveralls]

Coverage decreased (-0.01%) to 90.523% when pulling e184691 on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage decreased (-0.01%) to 90.523% when pulling 3b9eb3c on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage decreased (-0.005%) to 90.532% when pulling ca5ceb5 on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[kshedden]

See also #3579

[coveralls]

Coverage increased (+0.001%) to 90.538% when pulling fc2f21e on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage increased (+0.002%) to 90.539% when pulling fc2f21e on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage increased (+0.002%) to 90.539% when pulling fc2f21e on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage increased (+0.01%) to 90.547% when pulling 6c7f042 on kshedden:ctab_enhancements into 1c7f487 on statsmodels:master.

[coveralls]

Coverage increased (+0.009%) to 90.869% when pulling d8d8af9 on kshedden:ctab_enhancements into 359f08b on statsmodels:master.