Specify syntax of parsed strings in PICMI

[RemiLehe]

In a recent meeting, we discussed that we could point to the numexpr syntax:
https://numexpr.readthedocs.io/en/latest/user_guide.html#supported-operators

We may need to go through the list of operations in numexpr to make sure that all of them can be easily implemented by the supported PIC codes.

[dpgrote]

I checked all of the operators and they all work for numpy arrays, so using the numexpr standard should be compatible with Warp (which uses Python eval on the expressions relying on the operations being on numpy arrays).