add ventilation coefficient (Pruppacher & Rasmussen 1979 & Froessling 1938) + Reynolds number attr + air dynamic viscosity formulae + unit tests (incl. drop size changes upon ventilation-in-parcel-below-cloud vs. no-ventilation) + refactor of env vars __getitem__ logic

add ventilation coefficient (Pruppacher & Rasmussen 1979 & Froessling 1938) + Reynolds number attr + air dynamic viscosity formulae + unit tests (incl. drop size changes upon ventilation-in-parcel-below-cloud vs. no-ventilation) + refactor of env vars __getitem__ logic #2249