Allow differences between model and ref time coords

There are a few things stopping model data and reference data to be used together:

• time calendar (use cf_units change_calendar, see #42. Only support for conversion between standard, gregorian and proleptic_gregorian needs to be handled for now.) (see #42 instead)
• time bounds
• time long_name

Theses should be set in the reference data using the corresponding values in the model data.

Command line to expose problems (master branch):

midas -f -i -v -l info -t pr -p 1971-1980 -r /nobackup/rossby27/users/sm_carni/data/tmp/fixa_output/klimfab_test/day/pr/pr_NORDIC-11_SMHI-UERRA-Harmonie_RegRean_v1_SMHIGridClim_v1.1.0_day_*.nc -m /nobackup/rossby27/users/sm_joalo/data/issues/midas92/*.nc -o [YOUR_OUTPUT_LOCATION]