Introduce TracksCache and simplify _match_files(), add test

[zas]

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)

[phw]

LGTM

[Sophist-UK]

I appreciate the name already existed, but not so much of a cache (which you might expect to be persistent) but more of a lookup table created afresh every time _match_files is run and destroyed when it finishes (because it is a local object).

Would TracksLookup be a better class name?

[zas]

Would TracksLookup be a better class name?

No strong opinion about this, it really works as a cache, even if it's temporary.
Feel free to make a PR for a rename.

[Sophist-UK]

LOL - not that bothered.