use density formula from Formula in Pyrcel example

[slayoo]

currently, the density is computed with a hand-coded formula here:

PySDM/examples/PySDM_examples/Pyrcel/settings.py

Line 53 in f2f839e

return dry_air_density * (1 + self.initial_vapour_mixing_ratio)

while the same expression can be reused from:

PySDM/PySDM/physics/state_variable_triplet/rhod_thd_qv.py

Line 48 in f2f839e

return rhod * (1 + qv)