Redefine NHIST and N1850 to use Cloud2 tunings

[gold2718]

The KeyClim Cloud2 tunings are now the default for NHIST and N1850 compsets

• Added compset and test to reproduce the Cloud2 experiment
• Add namelist definitions, defaults and exceptions for clubb_meltpt_temp and clubb_dt_low
• Modify WBF effective factor for keyClim
• Update CAM external tag
• Added a top-level script, describe_version (taken from CESM), which documents the current code configuration.

[oyvindseland]

While I can certainly take over the issue, I think the critical part is the review and not the actual merge. So in order to save time I think it is better if I review the code and Dirk or Steve do the merge @MichaelSchulzMETNO

[oyvindseland]

This pull request is just adding a new cloud 2 SSP585 data set. There is no redefinition as in the title of the issue. Is it only a wrong title?

[oyvindseland]

Based on discussion with Dirk Olivie I think that this pull request is not relevant any more and can be cancelled. Given that the cloud2 settings should be default also the SSP585 should use the same settings as NHIST