Add option for Paleo diagnostics

[justin-richling]

New feature type

New plot and/or plot enhancement, New infrastructure or infrastructure enhancement

What is this new feature?

To continue the push to replicate the old AMWG diagnostics, adding the Paleo climate diagnostics will be helpful for the Paleo group.

There will be only minor code change, with the exception of needing to create continents for past climates. This will only be necessary when the ADF is comparing two cases that are in a past era, then the ADF will create the continent outlines via the LANDFRAC variable as a mask.

The other typical scenario is comparing a past era to modern pre-industrial (PI) case. In this scenario, the ADF will use the modern day continents for all plots. Thus, there will need to be a flag in the config file, ideally in the diag_basic_info section.

Assistance required?

No, I will make a PR when the feature is ready

Extra info

No response