pylint: variable names fixes

[darked89]

Mostly single letter variable names fixes pylint was complaining about. IMHO _col_num as a suffix is more readable than _field for input column indexes, but it is just a proposal.

[darked89]

Test failed because the gwas.py method had a spelling error: check_allelesarevaild' which I did change to: "check_allelesarevalid'

So it needs to be changed also in:

test/test_gwas.py
14:    g.check_alleles_are_vaild()
20:        g.check_alleles_are_vaild()

[mcgml]

Thanks @darked89 !