Changes for TXPipe

[joezuntz]

This is a work-in-progress of changes that I'm finding are needed to run the RedMagic selection within TXPipe. Ignore it for now, it's just for me to keep track. I'll update it as I find other issues.

So far:

• I needed an example redmagic calibration file for testing, so added an option (powered by an environment variable) to keep the one generated during testing
• The Configuration in RedMapper requires many options that are not needed as part of the RedMagic selection, only for earlier stages. I made it possible to use an alternative and more limited configuration object.