Implement multinominal regression for categorical phenotypes

[jrm5100]

See: https://www.rdocumentation.org/packages/nnet/versions/7.3-12/topics/multinom

Issues

• No compatibility with survey weights
• Not sure how to integrate with categorical variables (LRT)
• Need to calculate pvalues

Ideas

• Automatically set regression family and use this for categorical phenotypes?
• Functions to make use of external libraries easier, like generating formulas
• Implement it as a different ewas function, e.g. ewas_nn_multinom