Grid-based particle initialization and random init rewrite

[streeve]

• Both uniform and random creation from Cajita grids
• Updates the core library init to make it more similar (type tags and non-uniform box). Behavior changes to ignore particles for the variants with minimum particle separation (and therefore may not return the number of particles requested)
• Deprecates previous simple random init versions (these still loop until all particles have been created which is extremely slow)

Core versions should also accept ParticleList + user init functors done

[streeve]

@abisner this is finally ready for review

[streeve]

Now blocked by #654

[streeve]

@abisner this actually passes all tests. I removed the filter empty routine for now because of SYCL issues, but I think it's best to revisit that later

[streeve]

Failures unrelated