Add ch4 parameter to registry

Alexandra Claire Sidleyrequested to merge
alsidley/athena:ggHH_quartic into main

The ch4 parameter needs to be added in Generators/PowhegControl/python/parameters/registry.py (it was only added to Generators/PowhegControl/python/processes/powheg/ggF_HH_quartic.py in !77447 (merged)).

Tagging @bmoser

Edited by Alexandra Claire Sidley

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