Enable feature filtering (fdge, fpca) to restrict feature types used in analysis

[lianos]

We may want to restrict the feature_universe to a subset of genes/features even before further abundance (or other) based filtering happens.

For instance, you may want to restrict a DGE or PCA to use only protein_coding genes -- by convention these are stored in the meta column of the feature information, but you could also imagine different ways to filter features.

1. Let's get the feature_universe parameter into fpca (and other places); and
2. Think of clever ways to pass a feature tibble in there from the shiny interface.

[lianos]

This is linked to #20