Add parameters to control max iterations and tolerance in logistic regression fit

[henrydavidge]

We should allow the user to configure these parameters, especially for the approximate Firth fit. cc @bboutkov since I believe this came up in your tests.

logistic_regression definition: https://github.com/projectglow/glow/blob/master/python/glow/gwas/log_reg.py#L24-L33

[bboutkov]

Thanks for opening this. I have the code ready and it seems to be working. Unfortunately we have some updates to our open source contribution policies that I need to review, but should be back here shortly.