Add variation scenarios

[phackstock]

This PR allows scenarios with a _[a-z] suffix to be submitted.
The reason I opted for this implementation and not for some wildcard matching is that we'd need to first merge IAMconsortium/nomenclature#397, issue a new release of nomenclature and update our processing pipeline.
This way the new "suffix" scenarios can be submitted as soon as this PR is merged.

[danielhuppmann]

I guess you discussed this at length with the modelling teams. I think this “_a” is bad style and bad strategy but ok to merge if they insist.

[danielhuppmann]

I’d really like to stop mixing spaces and underscores - and suggest to use square brackets for variations similar to what we do with variable-specs…

Suggested change

	- "{SSP} - {ScenarioMIP}_{Option}":
	- "{SSP} - {ScenarioMIP} [{Option}]":

[danielhuppmann]

Using letters without explanations seems like a really bad strategy…

[volker-krey]

To be honest, I am not worried about with or without underscore. Given that Keywan communicated it with underscore in the meeting, changing it now may cause more harm than it does good. So would suggest to stick with the underscore to limit confusion in the limited time there is for submission. Also, this is purely for internal purposes and these variants will not become public as the a, b, c, ... nomenclature does not have any meaning.

[volker-krey]

Looks good to me as a pragmatic way forward. To avoid confusion would suggest to stick with the underscore in the name as communicated in the ScenarioMIP online meeting on 20 September.

[phackstock]

Seeing as the timeline is tight and this is the agreed upon naming convention from Friday, I'll go ahead with the merge.