ECMWF HINDCAST grib reader

[ecasellas]

Unimodel should read ECMWF HINDCAST grib files. A function named read_ecmwf_hindcast_grib can be added to unimodel/io/readers_nwp.py. To obtain metadata, _get_ecmwf_metadata may be used.

[sotis-am]

The ECMWF hindcast grib files don't have the same file name format as the ECMWF-HRES or ECMWF-ENS. Usually the valid time part is in the form of mmddhhii where mm is valid month, dd is valid day, hh is valid hour and ii is valid minute (https://confluence.ecmwf.int/display/DAC/File+naming+convention+and+format+for+real-time+data). However, for the ECMWF hindcast the valid time is in the form of mmdd____ (note the underscores instead of hh and ii).

Instead of adding a new reader, we can use the generic read_ecmwf_grib modified in #6 and change the valid_time parameter for valid_month, valid_day and valid_run in the importers module.