Custom Particle Update (ver 2)

[haykh]

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[haykh]

@LudwigBoess a few things that i think should be fixed:

• There is really no reason not to just have:

  struct NoCustomPrtlUpdate_t {};

  I really don't see why we need a template with an empty operator(). especially given that in the example pgen, the template is directly on the operator() instead.

• this is not very safe (in pgens/examples/custom_particle_update):

     template <class Coord, class PusherKernel>
     Inline void operator()(index_t p, Coord& xp, PusherKernel& pusher) const {

  should use coord_t<D>. also, again, there is no reason the template is not on the class itself.

• in the same pgens/examples/custom_particle_update, the coordinate conversions are incorrect (i think). i + di is not the same as the x coordinate, as they are in different coordinate systems. and likewise, (i + di) / beta will not be equal to time.

• why do we give access to xp at all? isn't it redundant? xp_Ph would make more sense.