Explanation/example for ParticleBinning units

[mccoys]

The units of ParticleBinning output from happi is not straightforward. Some explanation is given at the end of this section:
https://smileipic.github.io/Smilei/Use/post-processing.html#open-a-particlebinning-diagnostic

But it could be better explained, possibly with an example.

In addition, there may be a better way to do the calculation in the case of AM geometry. Currently, happi divides the result by Lx * Ry^2. But it might be better to divide by pi * Lx * Ry^2. That works when there are no axes along y or z. If there is one of those axes used, maybe the choice would be 2 * Lx * Ry.

[mccoys]

@Z10Frank Any opinion on the AM case?

[mccoys]

More explanation given now: 2e7d5b6

Still need to resolve the AM case

[beck-llr]

I think the current behavior makes sense. No need to change it but the documentation could be improved by mentioning the specific case of the AM geometry.
Another very useful addition to the documentation would be to give some hints as to what happens in the case of non standard axis. Typically the case in which the chosen axis is a function returning the radial position.