feat: add new begin_at_strategy argument and split dataset_obj()

[kakhahmed]

I am considering this as a Draft until the pycalibration MR is fully reviewed

• Expose begin_at_strategy calcat argument to switch between prior and closest strategies
• Split dataset_obj() deriving path part to different function, as it can be usefull to use externally

This MR is related to an already almost available change in pycalibration:https://git.xfel.eu/calibration/pycalibration/-/merge_requests/1035/

This is related to that issue as well : https://git.xfel.eu/calibration/planning/-/issues/200 of adding begin_at_strategy to calibration_constant_versions/get_by_detector_conditions API

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.06%. Comparing base (9e7914f) to head (9d1182a).
Report is 50 commits behind head on master.

Files with missing lines	Patch %	Lines
src/extra/calibration.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   71.07%   71.06%   -0.01%     
==========================================
  Files          23       23              
  Lines        2724     2730       +6     
==========================================
+ Hits         1936     1940       +4     
- Misses        788      790       +2     

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[kakhahmed]

This MR is ready to review from my side. The coverage tests are a bit rough on me :(

I see that there are no SingleConstant test methods, therefore I didnt try adding tests for the new file_path method