Initialize particles only where density is non-zero

[n01r]

Currently, particles are initialized everywhere in the box inside the bounds of zmin, zmax, xmin, ... if they are set.
Otherwise, these bounds are expanded to the full box size, which can lead to running out of memory quickly when very dense, spatially limited targets (e.g. foils, spheres, etc.) are initialized.

Following discussions from 2022/08/23 this is identified as a potential topic for the September hackathon.

Approaches for solving this:

• evaluate the density function twice and create an iMultiFab mask for non-zero density (preferred)
• Find a global bounding box for particles
• Find a local (per box) bounding box for particles

[ax3l]

tentatively assigning @atmyers for this, potentially hackathon; generally high priority for all non-trivial (non-uniform) simulations :)