Store the genes that were included in the compendium used to generate the reference distribution

### Context

#488

### Problem or idea

We should store/keep track of the genes that were included in the compendium used to generate the reference distribution. As I note in the "future thoughts" section of that issue

> QN _only_ for genes that were used to derive the initial quantiles and then interpolate in between. This avoids a case such as HGU133A and HGU133B samples getting the same values which is likely to be wrong.

### Solution or next step

Add this to the QN reference distribution step

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Read issue #488 and locate the QN reference distribution step mentioned here; the issue does not name files or tests. Determine where the compendium genes are selected and stored, then verify that the genes used to derive the initial quantiles are retained with the reference distribution.

Written by the indexing model from the issue text.