WIP: Generalized chromatic utilities

[mtlam]

This PR will generalize the dmxparse and dmxstats functions in pint.utils. The original functions will remain as convenience functions, e.g., dmxparse() calls chromatic_parse(prefix="DM"). The naming conventions follow the current implementation of utils.xxxselections(). Comments welcome!

• dmxparse generalized to chromatic_parse
  • Tests for dmxparse verified.
• dmxstats generalized to chromatic_stats
  • Tests for dmxstats verified.
• Updated CHANGELOG-unreleased.md

[mtlam]

I wanted to add a separate comment from the top. I would argue that in utils.xxxselections() that prefix="DM" is confusing because as a prefix parameter, the prefix part is "DMX" and not "DM". I think this should be update in this direction rather than kept in its current format, but will default to whatever people think is best.