Add an "emissions diagnostics" processor

[danielhuppmann]

This PR transfers the script for the Scenario Compass to compute emissions-diagnostics indicators and implements it as a nomenclature Processor subclass.

The idea is that this can be imported in a workflow as

from scenariocompass import EmissionsDiagnostics

emissions_diagnostics = EmissionsDiagnostics()

and then either in a longer processing workflow simply calling

df = emissions_diagnostics.apply(df)

or as a stand-alone (atomic) workflow like

df = pyam.read_ixmp4(platform, **emissions_diagnostics.input_data)
df = emissions_diagnostics.apply(df)

and then write only emissions_diagnostics.output_meta back to the platform.

Any thoughts about the design?

[dc-almeida]

I don't really have the domain knowledge to comment on whether what the processor is doing makes sense, but on the code/design side, I'd say, it would be good to have docstrings for the methods/functions, and if there are any variables/regions that are mandatory for processing to work, then to add some Pydantic validator to check that.

[phackstock]

Looks good to me in principle a few small comments and suggestions in line below.

[danielhuppmann]

Thanks for the review comments - I added tests to make sure that either an error is raised when the wrong units are used or that df isn't altered if none of the relevant datapoints (or a short time horizon) are given.