Resolve "Bend2D does not take particle charge into account"
This MR:
- takes the particle charge into account for Bend2D (closes #545 (closed)), RBend and RBend3D
- fix code order in RBend3D: angle has preference over field (see http://amas.web.psi.ch/opal/Documentation/master/OPAL_Manual.html#sec.elements.bend)
- cleans and refactors code