feat: add `pindex` to `InclusiveKinematicsVars` #889

Job	Run time
Linux (minver) / Download test data	2s
Linux (minver) / Build Minver Ruby	16s
Linux (minver) / Build ROOT	15s
Linux (minver) / Build HIPO (noROOT)	13s
Linux (minver) / Build HIPO (withROOT)	19s
Linux (minver) / Iguana (cpp)	4m 47s
Linux (minver) / Iguana (noROOT)	1m 45s
Linux (minver) / Iguana (python)	6m 16s
Linux (minver) / Iguana (fortran)	2m 51s
Linux (minver) / Final	1s
Linux (minver) / Collect webpages	0s
Linux (minver) / Deploy webpages	0s
Issue bot	0s
	16m 45s

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