Add flexibility for halo model choice

[oashour]

The parameters of the currently-implemented standard halo model should be updated to match the recommendations of this white paper https://link.springer.com/article/10.1140/epjc/s10052-021-09655-y (they are very slightly off)

The code should also be slightly refactored to make it easier to use other models for the velocity distributions (e.g., the SHM++ mentioned here https://link.aps.org/doi/10.1103/PhysRevD.99.023012