Tying features to modalities in joint-RPCA

Is there a way to easily tie which feature came from which modality at the moment? As far as I can tell, the current implementation stores all features across tables in an `OrdinationResults` instance. You can certainly make a bespoke mapping of feature:table but I think it would be useful to incorporate this into the codebase. Specifically, this would be useful for visualization if, for example, you wanted to plot a network and color the nodes by 16S/18S/ITS/etc.

Start by tracing the joint-RPCA results into the OrdinationResults instance and inspect how features from the input tables are currently stored. Determine how a feature-to-modality mapping could be exposed for downstream visualization; done means callers can identify whether each feature came from 16S, 18S, ITS, or another modality.

Written by the indexing model from the issue text.