[drias_daily] Unit error for snow precipitation

The function create_drias_database assumes mm/day unit for prsn the snow precipitation. The issue is that this data is in kg/m²/s as for liquid precipitation and potential evaporation.

So prsn should be transformed to in line https://forge.inrae.fr/umr-g-eau/radis.api/-/blob/main/R/create_drias_database.R#L194