Add selectnumrepresentations filter

[lilyminium]

This adds a new filter that selects data based on the number of representations of a particular filter. It didn't actually wind up that useful but could be later on, perhaps.

[codecov]

Codecov Report

❌ Patch coverage is 23.25581% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.63%. Comparing base (7b5714f) to head (e52fc4f).
⚠️ Report is 16 commits behind head on main.

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