New features!

1) Adds a beam_ion_effects module. Beam-ion interaction is implemented with weak-strong, strong-strong and Particle-In-Cell models.
New classes: BeamIonElement, IonParticles, IonAperture, IonMonitor
2) Adds space-charge module, implementing TransverseSpaceCharge class.
3) Adds electromagnetic fields module. Handles Faddeeva function and electromagnetic fields for space-charge and beam-ion interactions.

Other minor improvements are made to the code.