Molecular atmosphere category C calibration

Create a tool the performs category C calibration. It should request and download grib files from ECMWF via Copernicus CDS. That's the main difference with cat-B calibration. ECMWF provides grib1 files.

Potentially one could differentiate the exception scenario as well, in case of data unavailability (even though such a case never occurred in the admittedly limited request we have sent to Copernicus). Such scenario would chose the atmospheric model not based on date, but based on the latest selected model. We can implemented it once the DB implementation for the atmospheric calibration is finished.