Particle Binning: use actual gridspacing when available

[openpmd]

Particle Binning: use actual gridspacing when available

[RemiLehe]

Fixed by #132

[ax3l]

@RemiLehe did you intentionally change the milestone? I think we can tag 0.1.0 without it and add it later on since it adds completely new functionality (one can still start working on it earlier, but imho it is not required to get 0.1.0 out :) )

[ax3l]

An other comment: using the actual grid spacing (spatially) when available for binning and adding the correct particle shape are actually two independent tasks.

you might want to split the issue to reflect that :)

[RemiLehe]

Regarding the milestone: you are right, we should keep this for 1.0.0 -- especially since I just realized (when trying to implement it) that it will much easier once we have the unit consistency (#64) that will be part of 1.0.0