New rules for generating subsets of bigWig files

[cnluzon]

This PR adds ways to reduce the amount of bigWig files generated to the minimal set of meaningful ones. The idea is that most of the time we don't need to produce both scaled and unscaled files, so now there are rules to produce scaled treatments and unscaled (RPGC-scaled) controls, generally halving the amount of bigWig files generated. More specifically:

• quick: This rule existed previously, as opposed to full which also runs the fingerprints, but now it also includes only the minimal set of bigwigs. It is not currently the default, although I am considering making it the default final rule.
• pooled_only_minimal: Pooled replicates in groups.tsv but only scaled for treatments and unscaled for controls.

I have also included a get_all_controls function and now treatments and controls exist as global variables for legibility of these sets.