GRIB loading

I attempted to create some helper functions to load GRIB files with Start(). It can load one GRIB file with some restrictions:

• regular grid data (gridType: regular_ll)
• Global region
• selected time step
• no member dim, only [latitude, longitude, time]

Here are the files: https://earth.bsc.es/gitlab/aho/aho-testtest/-/tree/master/startR/GRIB

In the testing script, the first two cannot work now due to missing files. I found one file (the 3rd case) that Start() can successfully load.

Many remaining things to be solved, e.g., correct metadata, flexible dimensions, transform, load a region instead of global data.

The helper function uses "gribr" package. It's better to not have this dependency.

FYI @vagudets @erifarov