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

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