Enable FesomDataReader to have different source and target datasets

[kacpnowak]

Description

This PR implements separate source and target datasets for FesomDataReader.

It was also necessary modify masking strategies, as healpix cell can empty for the source but not for the target. Additionally there can be different number of tokens per cell. Different adjustments had to be made for each strategy:

• random: when mask is missing for each token, it's assumed to be False.
• healpix: when mask is missing it's for the entire healpix cell it's assumed to be False.
• casual: If mask is missing for an entire cell, it's assumed to be True for every token inside. If number of tokens in cell is different for source and target, source masked ratio is calculated and used for masking the same fraction of target tokens.
• channels: unsupported

Huge thanks for @shmh40 for all the help.

Issue Number

Closes #911

Checklist before asking for review

• I have performed a self-review of my code
• My changes comply with basic sanity checks:
  • I have fixed formatting issues with ./scripts/actions.sh lint
  • I have run unit tests with ./scripts/actions.sh unit-test
  • I have documented my code and I have updated the docstrings.
  • I have added unit tests, if relevant
• I have tried my changes with data and code:
  • I have run the integration tests with ./scripts/actions.sh integration-test
  • (bigger changes) I have run a full training and I have written in the comment the run_id(s): launch-slurm.py --time 60
  • (bigger changes and experiments) I have shared a hegdedoc in the github issue with all the configurations and runs for this experiments
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  • for config changes: the MatterMost channels and/or a design doc
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