Add defalut cx fields for MTG-IRS

[stemiglio]

This PR adds a list of cx fields that are used as default for MTG-IRS. The mods are particularly useful when processing MTG-IRS in the UKV as for this model configuration there is no MTGIRS.nl file

[ctgh]

Looks good to me. Do you eventually need to add a UKV namelist file?

[stemiglio]

Looks good to me. Do you eventually need to add a UKV namelist file?

Thanks for approving @ctgh. I am not planning to add a UKV namelist file, as the non-default fields (cloud_bulk, cl and cf, in UM notation) are only needed in VAR by the moisture incrementing operator, which is not used in the UKV (for now at least)

[mikecooke77]

Happy for this to go in. Do you plan to add a nl to jjaux for operational use or rely on this?

[stemiglio]

Happy for this to go in. Do you plan to add a nl to jjaux for operational use or rely on this?

For the global we do already have a namelist file for MTGIRS, see https://github.com/MetOffice/opsinputs/blob/develop/etc/global/cx/MTGIRS.nl. For the UKV we don't need more fields than the default ones, this is why I decided to create a default list of fields for MTGIRS rather than create a UKV namelist with default fields (I think this should only be used when you ask for non-default fields as done in the global). This means I was not planning to add a nl for operational use in the UKV.