Improve calculation of dEdX weights

[jbsauvan]

Currently, trigger dEdX weights are calculated from reco dEdX weights here. But since the reco dEdX weights are themselves derived from the individual layer weights (here), the way the trigger weights are computed is not exactly correct and can give large weights in transition regions.

One should derive weights directly from the layer weights instead.
Some reference slides on weight calculation can be found here.