Update FSC analyzer

[michael-pitt]

PR description:

PR Description

This update introduces several improvements to the FSC digitization and reconstruction workflow:

1. Dedicated FSC Data Structure
   A new header (FSCstruct.h) defines a dedicated struct where additional FSC-related variables can be stored.

2. Charge Calibration Using Calibration Constants
   The total measured charge is now computed using calibration parameters (currently hard-coded for validation; once verified, they will be moved to the conditions database).

3. Support for 50 ns Bunch-Spacing Reconstruction
   The reconstruction now handles 50 ns bunch spacing, properly accounting for signals from neighbouring bunches.

4. Extended fscdigi Output
   The fscdigi tree now includes:

• The calibrated charge per counter
• The total charge measured in each detector
• The global charge sum across all FSC detectors in one arm

Bunch-Spacing Configuration
The default settings of the module are optimized for PbPb data, which use 50 ns bunch spacing.
For datasets with >100 ns spacing (e.g., O-O, p-O, Ne-Ne, or any other HI run with long bunch spacing), you should switch to the full time-interval charge reconstruction.

To do this, update your configuration as follows:

process.load('HeavyIonsAnalysis.ZDCAnalysis.FSCAnalyzerHC_cfi')
# Use full time interval for charge reconstruction (for bs>100 ns)
process.fscanalyzer.doFullFitFSC  = cms.bool(True)
process.fscanalyzer.do50nsRecoFSC = cms.bool(False)

PR validation:

Tested with CMSSW_15_1_0_patch3 and recent HI data (run=399540)

If this PR is a backport please specify the original PR and why you need to backport that PR. If this PR will be backported please specify to which release cycle the backport is meant for: