Add MAGICC variables

[phackstock]

As MAGICC becomes more integrated into a number of projects I figured it might be a good idea to add all MAGICC variables to this repo.

[danielhuppmann]

FWIW, I think it might even be preferable to keep these variables in a separate repo directly attached to the climate-processing workflows - this way, the output of climate-processing is "together" with the expected variables. For validation, the variables could still be imported to a project-workflow repo (if necessary).

[phackstock]

@danielhuppmann, I like the idea. We could and probably should encourage this for all future post-processing steps. Each step should come with it's own set of defined output variables.

[jkikstra]

Okay sounds good, just let me know if/when you need anything from me.