P3: `D_gr` is less than `D_th` for some `F_rim`, `ρ_rim`

It appears that for some values of `F_rim` and `ρ_rim`, the expected relationship `D_th < D_gr < D_cr` does not hold.

In particular, `F_rim=0.93`, `ρ_rim=975.0` gives rise to `D_th > D_gr`. Code follows:

1. Using built-in functionality (needs #582):

```Julia
import CloudMicrophysics.Parameters as CMP
import CloudMicrophysics.P3Scheme as P3
FT = Float64
params = CMP.ParametersP3(FT)
F_rim = FT(0.93)
ρ_rim = FT(975)
D_th1, D_gr1, = P3.get_thresholds_ρ_g(params, F_rim, ρ_rim)
# 9.73e-5 9.69e-5
```

2. using previous iterative methods

```Julia
import RootSolvers as RS

FT = Float64
F_rim = FT(0.93)
ρ_rim = FT(975)

# Constants from CMP.ParametersP3
α_va, β_va = 0.018537721864540644, 1.9
ρ_i = 916.7

function ρ_d_helper(D_cr, D_gr)
β_m2 = β_va - 2
6 * α_va * (D_cr^β_m2 - D_gr^β_m2) / π / β_m2 / (D_cr - D_gr)
end
ρ_g_helper(F_rim, ρ_rim, ρ_d) = F_rim * ρ_rim + (1 - F_rim) * ρ_d
D_gr_helper(ρ_g) = (6 * α_va / π / ρ_g)^(1 / (3 - β_va))
D_cr_helper(F_rim, ρ_g) = (1 / (1 - F_rim) * 6 * α_va / π / ρ_g)^(1 / (3 - β_va))

P3_problem(ρ_d) =
ρ_d - ρ_d_helper(
D_cr_helper(F_rim, ρ_g_helper(F_rim, ρ_rim, ρ_d)),
D_gr_helper(ρ_g_helper(F_rim, ρ_rim, ρ_d)),
)

ρ_d =
RS.find_zero(
P3_problem,
RS.SecantMethod(FT(0), FT(1000)),
RS.CompactSolution(),
).root
# 193.4

ρ_g = ρ_g_helper(F_rim, ρ_rim, ρ_d)
# 920.3

D_th = (π * ρ_i / 6 / α_va)^(1 / (β_va - 3))
# 9.73e-5

D_gr_helper(ρ_g)
# 9.69e-5
```

Start with P3.get_thresholds_ρ_g and ParametersP3, reproducing the supplied F_rim=0.93 and ρ_rim=975.0 case and comparing it with the iterative RootSolvers calculation. Trace the threshold relationship and determine whether the expected D_th < D_gr < D_cr ordering is valid; done means the discrepancy is explained and the issue's expected behavior is resolved.

Written by the indexing model from the issue text.