Add initial driver for ACCESS-ESM1.6

[blimlim]

Closes the first part of #554

This PR adds an initial driver for ESM 1.6. This first version duplicates the ESM1.5 driver which until now has been used for ESM1.6. ESM1.6 specific changes (e.g. adding new configuration files, removing matm support) will be added in separate PRs in order to have clearer diffs.

The most contentious part could be the file and model names – I tried to follow what I'd seen elsewhere but let me know if there are different conventions that would be better.

• I chose the filename access_esm1p6.py based on accessom2.py, however added an underscore for clarity since they are used in other drivers like cesm_cmeps.py include them.
• For naming the model class, I used AccessEsm1p6, following other classes like AccessOm2.
• For the model-type I used access-esm1.6, following access-om2 in the OM2 driver, and also matching what is used in the spack packages.

[aidanheerdegen]

LGTM. I like your naming choices. Consistent with previous practice, and where it differs it improves clarity.

[blimlim]

Thanks @aidanheerdegen! Merged