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]

Please add tag-lists for the percentiles and the species, see https://github.com/IAMconsortium/common-definitions/blob/main/definitions/variable/emissions/tag_tier3_species.yaml (but you cannot directly re-use because of different units).

[danielhuppmann]

For consistency with the variable-notation guidelines.

Suggested change

	- AR6 climate diagnostics|Atmospheric Concentrations|CH4|MAGICCv7.5.3|10.0th Percentile:
	- AR6 Climate Diagnostics|Atmospheric Concentrations|CH4|MAGICCv7.5.3|10.0th Percentile:

[jkikstra]

Is this PR still meant to be merged?

And if so, I suppose this capitalisation be changed directly in iiasa/climate-assessment?
I don't have any capacity for this right now (it's a small update, but updating the test-data is a bit of a pain, and just need to do a quick check to make sure it wouldn't break anything in notebooks or other scripts, and backward-compatibility).

[phackstock]

For now, I wouldn't say it's super urgent. MAGICC variables are currently added after the variable validation so this PR is good to have for sure, but not essential right now.

[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.