Fix the Global attribute dac_qc_comment to list clear messages about variable issues

[leilabbb]

This is related to #318

The dac_qc_comment was added to the file's global attributes to list issues encountered when implementing the QARTOD tests.

• The issues related to non-standard standard_name and using standard standard_name by more than one variable must be rewritten to convey a clear message. Click the related code
• A dataset with an example of the issue:

dac_qc_comment "QC skipped forsea_water_practical_salinity: the same standard name is shared by , Skipped QC: density is an array of nans";

• The issues related to empty arrays need to be edited for clarity. Click the related code
• A dataset with an example of the issue:

dac_qc_comment "Skipped QC: temperature is an array of nansSkipped QC: conductivity is an array of nansSkipped QC: density is an array of nansSkipped QC: pressure is an array of nansSkipped QC: salinity is an array of nans";
``

• The issues related to the time array need to be reported: Click the related code
• A dataset with an example of the issue:

The statement must be added to complete the loop and included in the dac_qc_comment.