Python example of grid coordinates and updates to GitHub links

[mauzey1]

Resolves PCMDI/cmor#766

Adds example code and NetCDF output demonstrating the use of cmor.grid.

This was made in response to @ilaflott issue at PCMDI/cmor#766.

This PR also contains some changes to GitHub links. The branch name master is replaced with main for links to the CMOR repo. Links that point to sections of CMIP6_CV.py in the cmip6-cmor-tables repo have been replaced with permalinks to prevent the linked section from shifting when the CV is changed.

[durack1]

We probably need to start pulling out the direct references to the CMIP6 project, and rather start listing projects using CMOR. CMIP6 is effectively closed for registrations, with new models/experiments being defined in CMIP6Plus_CVs for now, and CMIP7_CVs online in the coming months. Then there is input4MIPs and obs4MIPs, amongst other projects (DRCDP, etc) using the software

[mauzey1]

@durack1 I agree with listing other projects that use CMOR aside CMIP6 but I think we should make that a separate issue for another PR.

[durack1]

@durack1 I agree with listing other projects that use CMOR aside CMIP6 but I think we should make that a separate issue for another PR.

Couldn't agree more - are you happy to do it (create an issue)?