decouple grid resolution and density tuning with “particles per cell” vs “particles per square meter”

[jakebonney10]

Both the weight-based seeding and uniform seeding routines currently interpret a single “particles per cell” parameter, even though it semantically acts like “particles per m².” This coupling makes it hard to tune density independently of grid resolution.

Impact:

Difficulty achieving predictable spatial density of particles when changing grid cell size parameter since its coupled to the density steps in the pf

Suggested fixes:

• change—particles_per_cell to particles_per_m2—so its decoupled
• Refactor the seeding and weighting code to use the appropriate parameter in each context

[jakebonney10]

This should be documented in read me for someone tuning grid cell size how it will effect particle density weighting