Updates for mip-cmor-tables

[mauzey1]

Resolves #709, #718

• CMOR will throw an error if it processes a JSON file that contains null values. Used to avoid segfaults.
• When processing attributes in variable entries, any JSON list values will be converted into a space-separated lists string. Used for the dimensions and modeling_realm variable attributes.
• Modeling realm strings were being split by spaces and only had the first word found stored in the realm attribute of the file. This has been fixed by storing the whole realm string.
• Bumped version from 3.7.3 to 3.8.0 to allow for the use of mip-cmor-tables

[durack1]

@mauzey1 thanks!

[mauzey1]

I plan to incorporate a CMIP6Plus test into this PR that will also include having the mip-cmor-tables as a submodule. I will also need to bump the version number in order for CMOR to work with the mip-cmor-tables. Maybe CMOR 3.8.0a or something along those lines.

[durack1]

I plan to incorporate a CMIP6Plus test into this PR that will also include having the mip-cmor-tables as a submodule. I will also need to bump the version number in order for CMOR to work with the mip-cmor-tables. Maybe CMOR 3.8.0a or something along those lines.

This sounds great to me. I figure that 3.8.0, 3.8.1+ will be pointing to the mip-cmor-tables, and prior versions have more hardcoded connections to the cmip6-cmor-tables, but have also been used with input4MIPs-cmor-tables and obs4MIPs-cmor-tables. So let's keep the versioning following the 3.8.x naming, rather than adding an additional identifier