add action for estimating (relative) abundances MAGs/MAG-based taxonomic profiles

[nbokulich]

When performing taxonomic profiling of MAGs, abundance information currently is not preserved. This is needed to then estimate the (relative) abundances of MAGs/taxa.

This can be done in a few steps:

1. map reads to MAGs.
2. normalize by genome length, e.g., calculate RPKM or similar. c.f., bracken uses genome length to calculate relative abundances of taxa when doing read-based profiling.

This action should probably be independent to taxonomic classification, i.e., to output a feature table of MAG abundances per sample.

Some useful reading

1. Challenges in benchmarking metagenomic profilers describes the differences between sequence abundance and taxon abundance for estimating relative abundances in metagenomes.
2. mOTUs2 uses a single-copy marker-gene approach for accurately estimating taxonomic abundance from reads, but gives a good overview of the different approaches and a comparison vs. MAGs.

[misialq]

Closed by #177