Add Extended Description attribute to the Activity class

[charliepascoe]

The original purpose of the experiment documentation for CMIP6 was to provide modelling groups with a self contained resource to assist them with setting up simulations for CMIP6 and also to gather structured information from modelling groups about how their models were configured to run those simulations.

However, ES-DOC resources are also destined for use outside of the CMIP6 ecosystem e.g. in the Copernicus Climate Change Services (C3S) portal. In the C3S context the short-hand used in the experiment "description" and "rationale" fields are not sufficient to describe the CMIP6 experiments in a way that can be understood by non-expert users.

This proposal will add an optional "Extended Description" attribute to the activity class.
The Extended Description can be considered as something similar to an abstract. It will allow for a more complete description for experiments so that they can be understood by non experts and understood outside of the context of CMIP6 ecosystem. The attribute will be optional (0,1) to recognise that many of the experiments in MIP activities are likely to only be consumed by the modelling groups who contribute to the MIP.

@bnlawrence I'm at-ing you to keep you in the loop with this one.

[davidhassell]

This new property would be on activity.activity (https://github.com/ES-DOC/esdoc-cim-v2-schema/blob/master/activity_classes.py#L10)