Make units of P3 non-dimensional

[rorlija1]

Units of α_va_BF1995 must contain β_va_BF1995. Ideally, we would add a reference particle dimension D_0 in the mass as a function of maximum particle dimension relations as implemented in CloudMicrophysics 1-moment scheme so that the units of α_va_BF1995 are kg.

Similarly, the units of 𝛾_M1996 must contain 𝜎_M1996. A reference particle dimension should also be added to the area power laws to adjust the units. Additionally, neither Mitchell 1996 nor Morrison and Milbrant 2015 provide, to my knowledge, documentation of the units of length for dimension D nor the units of projected area A, so more investigation into that is needed.

In tandem with this issue, see the corresponding CLIMAParams issue.