Improve functionality for Gaia-only targets.

[geordie666]

This PR includes a range of improved functionality for Gaia-only targets:

• Deredden the Gaia colors when selecting Gaia-only standards.
  • Include a general function to de-extinct Gaia magnitudes.
  • Add E(B-V) as an EBV column in the supplemental targets for downstream spectrophotometric calibrations.
• Add a flavor property to the bitmask for the secondary targets.
  • This flavor can be "SPARE", "DEDICATED", "SSV", "QSO" or "TOO".
  • Also add a unit test to check the allowed values of flavor.
• Update the priorities for "dedicated" secondary targets.
  • This is facilitated by the fact that such targets can be distinguished from "spare" targets using flavor.
• Include PHOTSYS of "G" for targets that are selected using only Gaia.
• Fix some minor bugs in directory structures and columns in pixweight files that arose due to changes in the data model.
• Add code to scrape the Gaia EDR3 flat (csv) files from the web.
  • Also code to reformat them as FITS files, and by HEALPixel.

[geordie666]

I need to run new targets to go back on-sky tomorrow. So, I'll merge this and make a new tag after tests pass (likely early tomorrow morning).