gwas.py: fixed gwas demo run and IID column position

[ttfiliz]

Fixes #230, #232

Changes proposed in this pull request:

-Renamed Plink2 case/control GWAS output files to *.logistic (from *.firth and *.logistic.hybrid)
-Added --firth option to gwas.py

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[espenhgn]

Thanks @ttfiliz! Looks good. Can you bump the version.py file and update the CHANGELOG.md for this to be merged?