Move `get_window_size` factor of merged_s2s as untracked configuration

[dachengx]

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What does the code in this PR do / what does it improve?

The original factor is 5. This PR moves it to an untracked context configuration, because in reprocessing we might want to change it, but better without changing the source code.

It is trying to resolve: https://github.com/AxFoundation/strax/issues/490

This PR will not change the result of MergedS2s. It will not change the lineage either.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• [ ] Update the docstring(s)
• [ ] Update the documentation
• [x] Tests to check the (new) code is working as desired.
• [ ] Does it solve one of the open issues on github?

Notes on testing

• Until the automated tests pass, please mark the PR as a draft.
• On the XENONnT fork we test with database access, on private forks there is no database access for security considerations.

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[coveralls]

coverage: 93.564% (+0.02%) from 93.541% when pulling 81adf1d4ff5560bf6e073f68069d4ab86ed6ff0b on merged_s2s_factor into a95280595c9b9cd76ab9839121c5b9f2a24df954 on master.

[dachengx]

Thanks, both of you!