CMIP6 UDA catalog

[aradhakrishnanGFDL]

The UDA catalogs for CMIP6 can be generated with a config file in this version.
The config-cmip template needs an update to match the columns we have agreed to, though not 1:1 with esgf catalogs.

The change to the esgf catalogs can be made if necessary at a later stage.

This issue helps with the esnb development and further enhancement and testing with CMIP6 datasets, serving as an example for other UDA collections.

[aradhakrishnanGFDL]

How to test:

./gen_intake_gfdl.py --config ../configs/config-cmip.yaml

(or use fre-cli)

This version also presents a ./gen_intake.py so we can transition to that, given the applicability of catalog builder to not just GFDL at this point. The original idea was to have users develop their wrapper scripts for each case like CESM,can be revisited later and focus on ease of adoption of tool right now