Improve checking of global attributes in CMOR (enhancement)

[ehogan]

Following #88 (which was closed without resolving), using CMOR 3.4.0, I note the following with regards to the required global attributes:

Attribute	Form	Checked?	Replaced?	Logged?
activity_id	CV	No	No	No
experiment	CV	No	No	No
experiment_id	CV	No	No	No
forcing_index	int	Yes	N/A	N/A
further_info_url	CV	Yes	N/A	N/A
grid	Free form	No	No	No
grid_label	CV	No	No	No
initialisation_index	int	Yes	N/A	N/A
institution	Registered	No	Yes	Yes
institution_id	Registered	Yes	N/A	N/A
license	Some required text	Yes	N/A	N/A
nominal_resolution	CV	Yes	N/A	N/A
physics_index	int	Yes	N/A	N/A
realization_index	int	Yes	N/A	N/A
source	Registered	No	Yes	No
source_id	Registered	Yes	N/A	N/A
source_type	CV	No	No	No
sub_experiment	CV	No	No	No
sub_experiment_id	CV	No	No	No
variant_label	CMOR	No	Yes	No

Comparing this with Table 3 (page 18) in the CMIP6 Global Attributes document, I believe the following changes should be made to CMOR for CMOR to be consistent with this document:

• replace and log activity_id (available via experiment_id)
• replace and log experiment (available via experiment_id)
• check experiment_id against CVs
• check grid_label against CVs
• log source replacement
• check source_type against CVs
• replace and log sub_experiment (available via sub_experiment_id)
• check sub_experiment_id against CVs
• log variant_label replacement

Note: without these checks, it is possible for me to create the following file without issues:

output/CMIP6/a/MOHC/UKESM1-0-LL/c/s-r1i1p1f1/Omon/masso/g/v20190121/masso_Omon_UKESM1-0-LL_c_s-r1i1p1f1_g_201001-201005.nc

where a, c, s and g are activity_id, experiment_id, sub_experiment_id and grid_label, respectively.