DM-22802: Changes requested by the DRAGONS team

[timj]

This is a rebased version of #44 . It's slightly different than before because python 2 is no longer supported and the code was reformatted with black. The OrderedDict usage was dropped since dict is now ordered and I also protect the AstroData import.

Obvious things we need to think about:

• How do we make the AstroData supported type something that a user can add without it being hard-coded? Knowing that AstroData.filename should be used seems wrong so we may need to think about whether __str__ can be used or if we have a method that a new type must support.
• The python/lsst -> DRAGONS file name mangling.

cc/ @chris-simpson

[timj]

@chris-simpson / @KathleenLabrie what is your current status regarding pex_config usage? Have you made additional changes since this PR was made? Have you stopped using it?

[chris-simpson]

We're still using a fork. I've make a few more changes I'd have to dig out. Some are associated with our command-line usage (for example, having ListField accept and parse a comma-separated string).

[timj]

@ktlim I've had a go at rebasing this again.