dict=True option returns HaloDict object

from simulate_screen and simulate_uniform
HaloDict.ecf returns halo fraction as a function of energy
cleaned up (HaloDict.ecf returns halofrac as a function of energy)
added PAH option

added PAH option to sigma_scat
Can alter scattering model in simulations

simulate_screen and simulate_uniform functions now have a scatm keyword

Default is sigma_scat.Scatmodel()
Added rho keyword

Added rho keyword to uniform_halo and screen_halo

Also updated comments to reflect new keywords
Deleted loop_status

function no longer in use