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