Add user-targeted data input validation functions

[alkaZeltser]

We currently have comprehensive input data checking for all data submitted to the apply.polygenic.score function. This happens via helper functions, but they are still quite specific to apply.polygenic.score params. It would be useful to refactor these or write similar functions intended for use directly by the user to validate their input data (genotype, pgs, phenotype) prior to using apply.polygenic.score, and without having to include all the other params requested by apply.polygenic.score