Adding Boltzmann distributed electrons

Warp can include a region in which Boltzmann distributed electrons are added. This can be used to simulate a plasma source. The region can be limited by defining

w3d.xbemin = ...

w3d.xbemax = ...

w3d.ybemin = ...

w3d.ybemax = ...

w3d.zbemin = ...

w3d.zbemax = ...

If these limits are not set, they default to +-LARGEPOS.

The distribution itself is given by the following parameters:

w3d.electrontemperature = ... # in eV

w3d.iondensity = ... # in Coulomb/m^3

w3d.plasmapotential = ... # in V

If ions are created inside this plasma, the voltage of the injection source should be the same as the plasmapotential

top.vinject = w3d.plasmapotential