Cluster pgen Cleanup

[forrestglines]

We identified a few aspects of the cluster setup in #2 that could be cleaned up, although they are low priority

• [x] Unify the calculation mean molecular masses from helium mass fraction
• [ ] Simplify/avoid the passing around of hydro_pkg for the different cluster functions
• [ ] k_0 and k_100 have bad orders of magnitude, <-100. This could be partially rectified by dividing these quantities by mean particle mass
• [x] ~~Make determining the radial points where the 1D HSE profile is integrated more robust without the r_sampling parameter~~