Develop a script version of the "check_gribs" Olive experiment

Cf.
http://cxolive1.cnrm.meteo.fr:8181/swapp_entry/lenny/Olive/Browse/home/coope/alex/experiments/check_gribs/GMYR/gribs/

This single task runs the "fields_in_file" expert on GRIB files from an experiment compared to a reference one, in order to check the differences between post-processed fields.

To be implemented in a standalone Vortex script (no mkjob).