combine covars with PCs if --covars and --pca called

[dvitale199]

Currently, --covars and --pca used together results in defaulting to covars. we want to be able to run --pca and concat the pcs with the existing covars

Describe the solution you'd like add method to check if covars contains PCs already (just in case) and then generate pcs, and merge with input covars to use in the association