change N_chunk to URLConfig in peak_positions_cnf

[juehang]

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

The PeakPositionsCNF plugin requires a N_chunk parameter. This PR changes the N_chunk parameter to be a URLConfig, instead of a hard-coded number.

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:

• [x] Update the docstring(s)
• [x] 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: 89.85% (+0.02%) from 89.829% when pulling c383b5911c4598b37d410adcd9116f00c2bf5fe2 on cnf_Nchunk_URLConfig into f9c689f4b95958d5288f75de920dc6a6bddd7214 on master.