Move EXTRA_VARIABLES to picard/util/tags.py

[zas]

Suggested by @phw in #2633 (review)

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

• JIRA ticket (optional): PICARD-XXX

Solution

Action

Additional actions required:

• Update Picard documentation (please include a reference to this PR)
• Other (please specify below)

[zas]

@phw @rdswift Perhaps we should also rename it, do you have any ideas for a better name?

[phw]

Perhaps we should also rename it, do you have any ideas for a better name?

Maybe KNOWN_VARIABLES? Because there is an endless amount of possible variable names, but this includes certain ones that are used internally by Picard and are generally known.

[zas]

Perhaps we should also rename it, do you have any ideas for a better name?

Maybe KNOWN_VARIABLES? Because there is an endless amount of possible variable names, but this includes certain ones that are used internally by Picard and are generally known.

Not sure about the name, I'll merge this PR, we can rename later.