Add metric avg batch r2

[LuLeom]

Describe your changes

• Added average_batch_r2 metric : This metric quantifies, on average, how strongly the batch (B) explains the variation in each measured marker across all paired samples. There are two variation to the metric:
  1. average_batch_r2_global does not take into account cell-type
  2. average_batch_r2_ct does
• Added helper function remove_unlabelled

Checklist before requesting a review

• I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • Breaking changes
  • New functionality
  • Major changes
  • Minor changes
  • Bug fixes

• Proposed changes are described in the CHANGELOG.md

• CI Tests succeed and look good!