support second order accuracy in time

[alecjohnson]

iPic3D is currently only first-order accurate in time. Second-order accuracy in time requires that particles be pushed with field values that approximate time-averaged values and that the implicit susceptibility be calculated with field values and densities that approximate time-averaged values.

To achieve this, I would introduce support for options something like:

  PushWithBatTime = .5 # default: 0
  PushWithEatTime = .5 # default: 1
  ImplSusceptTime = .5 # default: 0

More details are posted here.