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

• 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.
• 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:

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