ZDC development branch to be merged into 21.0

Merge development branch "21.0-ZDC-OnlyCopyZdcAnalysis" into 21.0. The branch contains the latest edits from 21.2. The 21.2 version was last updated and MR-approved 3 months ago. Here is the link: !35558 (merged)

These changes include:

1. Updates to pulse fitting to improve performance for 2018 PbPb
2. Add repass for the pulse fitting to reduce the effect of the low energy peak (lower than 1n peak) which is due to the large pile-up events.
3. Improve the baseline matching performance for delayed and undelayed samples.
4. Change the 2018 fit function to "FermiExpLinear".
5. Update standalone test files in "tools" folder.
6. Fraction calculation for LGOverflows.

Now we are updating the 21.0 ZDCAnalysis to be consistent with the latest version on 21.2.

@steinber