Refactor BORG nbody scripts

Changes:

[x] Cleaning up and organization of run_density functions in BORG nbody, making helper functions, etc.
[x] Replace vfield_CIC written in pmwd with rho_and_vfield written in Pylians, in all nbody scripts
[x] Adds get_particle_mass to go from cosmology -> Mparticle, for making density field
[x] cosmo_to_astropy now takes either a list of parameters or kwargs. It also flexibly takes multiple input types.

maho3 / ltu-cmass