[WIP] Tune depth at which particles are removed, in the EB

[RemiLehe]

Motivation:
In order to avoid artifacts whereby particles leave a spurious charge behind as they cross the EB (seen in plots of div(E)), it is sometimes desirable to remove particles only once they have moved deep enough inside the EB that their shape factor is entirely contained in the EB.

Here is an illustration of the issue occuring with div(E) (right picture): there is non-zero div(E) staying at the position where particles hit the EB

TODO:

• Add automated test
• Scale depth by the cell size ; update documentation accordingly. Indicate that the default is 0.
• Update code that finds the particle intersection with the EB, so that WarpX only searched for the intersection at the timestep when particles cross the EB (not at later timesteps)