Natural Gas Updates

[trevorb1]

Closes #399, #400

Changes proposed in this Pull Request

In this PR I have:

• Integrated methane tracking. Gas production facilities are now modelled as stores + links rather than generators.
• Applied natural gas import/export constraints, tied to modelled weather year
• Updated relevant emission accounting constraint equations for ch4 tracking

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Changed dependencies are added to envs/environment.yaml.
• [x] Changes in configuration options are added in all of config.default.yaml.
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv.

[trevorb1]

To be done before merging:

• [x] Correct international natural gas import/export constraints
• [ ] ~~Add config option for allowing range of imports/exports (ie. extend limits by x%)~~
• [x] Update natural gas plots

[trevorb1]

@ktehranchi; can you please confirm neither of the changes in this commit are breaking changes for you?

• The log path change reverts the change from this commit. I was getting a NotADirectory error if left as is. Maybe this is a snakemake version thing though?
• The add_co2_emissions() I dont think is needed at the end? The CCS techs are captured in the attach_newCarrier_generators function, I believe. The parsing of the - causes quite a few issues for sector names, as many use -. Removing the catch all add_co2_emissions() at the end bypasses the sector issues, though.