add end_poles to Undulator class (draft)

[andre-trebushi]

Added end_poles to Undulator class as it was an argument of track4rad_beam(), calculate_radiation() etc. functions. Now you can switch on/off end_poles for your undulators in MagneticLattice(). This made to emulate phase shifters consisting of, basically, 2*undulator periods or 1.5 periods (Lu, H. H., Li, Y. & Pflueger, J. The permanent magnet phase shifter for the European X-ray free electron laser. 2009) where end_poles are not needed, see figures in attachment.

Some other minor changes also were introduced.

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