post_filter_na_preds()

[dajmcdon]

create a post processor that removes NA predictions

[ChloeYou]

It seems like this is already semi-implemented in https://github.com/cmu-delphi/epipredict/blob/39-post_filter_na_preds/R/layer_naomit.R by Daniel.

Although unit tests for it hasn't been added?