Add rules for generating gene expression & count tables

Description

For downstream analysis (most importantly for differential gene expression analysis) it would be desirable to generate summary tables for expression (TPM) and projected raw counts for each feature (gene, transcript, transcript region and/or exon).

Possible implementations

A single table could be created individually for each sample and feature type. Alternatively or in addition, a single common table could be created for all samples in a given run, each for expression and projected raw counts and for each of the feature types.

Additional context

In this regard it might be worthwhile to discuss the implementation of a results database that allows, e.g., to check the expression of genes of interest across all samples that have been run in the past.

Related issues

It is possible that #23 (closed) already addresses this issue. #30 (closed) includes a rule for collecting Salmon transcript expression estimates for all samples in a run and compiling a common table.