ClosedOrbitFinder/SigmaGenerator: Charge not applied correctly

Summary

The class ClosedOrbitFinder does not apply the charge correctly. It is currently only valid with a +1 elementary charge. If a user wants to find a closed orbit with electrons or other ions, the magnetic field is not correctly applied.

See the computation of bcon_m and check SigmaGenerator::initialize.

Possible fixes

Used PartBunchBase::getQ() in order to compute the quantities correctly.

CC: @ext-calvo_p, @snuverink_j