Diagnostics: Slice

[ax3l]

For diagnostics of particles and fields, we already have an option to limit the selected box of interest via lower_bound/upper_bound.

https://picmi-standard.github.io/standard/diagnostics.html#standard-pic-diagnostics

This is very general and useful, e.g., to zoom into a data-saving region of interest or creating slices.

For the later, which is a very common use case (slicing), we could add one more specialized argument.

cc @tsung1029 @tdalichaouch