Arbitrary date restart option does not apply to drifters

[HR-polar]

The new "arbitary" option for restart.type (issue #620) does not modify the output date for the OSISAF_Drifters.nc files. So while the Moorings.nc files start from the date specified in the config file with the "arbitrary" option, the OSISAF_Drifters.nc files start from the date in the restart files. This leads to a mismatch between the different outputs. With the "arbitrary" option, the desired behaviour is for the OSISAF_Drifters.nc files to start from the date in the config file, like for the Moorings.