Argo DOI monthly snapshot, core-Argo and BGC-Argo zipped files

In Argo DOI monthly snapshot, core-Argo and BGC-Argo are now compressed in two disctinct zipped files.

This is an update on ADMT action #15.
Keep DOI monthly download available with one click and one DOI.  Inside the download, split the GDAC holdings into two compressed files per DAC:  one for core and one for BGC files.  Include README file with naming conventions inside. Include all documentation for all User Manuals.

This change facilitates Core-Argo users data access who can ignore the voluminous BGC-Argo files.

The change was implemented from July 2019 monthly snapshot on: