Use eckit-geo directly to handle GRIB on unstructured grid

[sandorkertesz]

Description

When gridType == "unstructured_grid" instantiate an eckit.geo.Grid from the gridSpec to handle the field geography. For other grids still uses ecCodes for this. This was necessary to overcome the problems related to ORCA and ICON grids. See: https://jira.ecmwf.int/browse/ECC-2257

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