Add method to calculate the exposure

[fhagemann]

Closes #33.

In this PR, the method get_exposure is introduced, that takes LegendData, DetectorIdLike and a selection of runs (either DataPeriod & DataRun, just DataPeriod, or DataPariition as input to calculate the exposure for that given detector in that given selection of runs.

Example usage would be:

using LegendDataManagement
l200 = LegendData(:l200)
get_exposure(l200, :V00050A, DataPeriod(3), DataRun(0))
get_exposure(l200, :V00050A, DataPeriod(3))
get_exposure(l200, :V00050A, DataPartition(1))

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 12 lines in your changes missing coverage. Please review.

Project coverage is 39.05%. Comparing base (8e1da65) to head (6814279).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/exposure.jl	71.42%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   33.95%   39.05%   +5.10%     
==========================================
  Files          26       27       +1     
  Lines        1944     1987      +43     
==========================================
+ Hits          660      776     +116     
+ Misses       1284     1211      -73     

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[fhagemann]

I also wrote some tests (they also made me spot a bug --> 8629f59), but they require an update to legend-test-data (which I have requested in legend-exp/legend-testdata#22) and a new version with the updated LegendTestData afterwards.
Therefore, the file test_exposure.jl is not included in the standard tests, but should once everything is updated.

[fhagemann]

@theHenks This would be good to go now

[theHenks]

Let's merge than @oschulz . Looks great! Big thanks to @fhagemann