Update handling of coverage reference files

[marchoeppner]

This is a reminder for a future developer:

As of release 1.1.1, the pipeline supports reference panel coverages for two scenarios: the new default with an interval padding of 15bp and one for any other interval padding. These are specific to each panel, since the reporting script needs to match target names (which are not the same as the exome targets) from the per_target coverage file and the reference coverages.

However, additionally these coverages should be specific to the combination of exome kit AND panel definition. Right now this isn't the case, which works as long as only one exome kit is in use (xGen v2).

In the future, if new exome kits are established, there needs to be a way to address the combination of kit and panel coverage specifically.