Additional outputs and summaries following a sourceapp run

[bglindner]

As the documentation describes, SourceApp produces a dataframe of prokaryotic population fractions (if --use-geq was flagged) or relative abundances for each source in the database used. It would be helpful for users to have more than just this dataframe to assist with exploring their data.

Does it make sense to create a utility to summarize these extra outputs or include them in the main pipeline? If we include extra util(s), we need to output an object that captures the underlying data -- maybe just the CoverM output from step 4?