Add gas categorization.

[mikapfl]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Description in CHANGELOG.rst added

Description

Add a categorization for PRIMAP2 entities.

[mikapfl]

So far, only emission rates derived from openscm_units are included. What else do we need? PRIMAP1 contains a lot of categories with (for me) kind of dubious codes and definitions like CO2PERC: CO2 as Percent of sth.. Also, in contrast to PRIMAP1 in PRIMAP2 we do not need to pre-define all entities, so I would propose to just standardize the common ones with a clear definition, where homogeneous naming actually is useful.

So far I understood that we definitely need:

• emission rates of basic entities
• emission rates of gas baskets (which baskets?)
• socio-economic quantities (which ones?)
• What else? Temperatures?

[JGuetschow]

Looks good. I made some comments on possible additional fields (probably too much work as rest is generated automatically).
One more important comment is on gas basket entities and the entity hierarchy

[codecov]

Codecov Report

Merging #1 (56b0737) into main (dd1a325) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main       #1   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files           9       10    +1     
  Lines         685      692    +7     
=======================================
+ Hits          680      687    +7     
  Misses          5        5           

Impacted Files	Coverage Δ
climate_categories/__init__.py	87.50% <100.00%> (+0.40%)	⬆️
climate_categories/tests/test_gas.py	100.00% <100.00%> (ø)

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[mikapfl]

I narrowed down the focus now, simply to have at least the working subset right now. It includes only gases and mixtures so far, in a separate terminology. We can then add more (e.g. gas baskets and socioeconomic entities and whatnot) to a primap terminology which builds on the gas terminology.