WIP: Resolve ATR-22206 "Tm serial merge merged chains"
Adjusts merging script in ChainMerging.py to append all sequences associated to a given step to be merged whether they are from single chains or already merged chains.
The implementation of serial merging currently only allows for chains that have one menu sequence. However, this doesn't allow for chains like e10_g12_mu6, where the e+photon parts need to be parallel merged, after which the muon is serial merged.