Helper functions for parameters which use 0-based indexing

As mentioned in https://git.km3net.de/vkulikovskiy/refitvhe/-/merge_requests/8#note_71055 the indexing is messed up since our CSV files with the definitions use 0-based indexing and Julia is 1-based.

A solution to this would be providing helper functions which automatically add one to the index.

e.g. instead of writing

muon.fitinf[KM3io.FITPARAMETERS.JGANDALF_CHI2 + 1]

one should do

fifinf(muon, KM3io.FITPARAMETERS.JGANDALF_CHI2)

(btw. we should also export FITPARAMETERS so that KM3io. is not needed)

What do you think @vkulikovskiy ?