Index snapshots by planning horizon

[trevorb1]

Closes #335

Changes proposed in this Pull Request

In this PR I have changed the indexing logic to be the planning horizon instead of the profile year. This includes:

• Adding multi-index to all snapshot data (regardless if it is single horizon or not) in build_base_network
• Updating demand to export planning horizon data (note, not scaled data yet) - the expand_demand function is a little awkward, but it will allow for addition of scaling demand relatively easily.
• Updating data in add_electricity to account for planning horizon (ie. renewables, fuel prices, ect...) through updated broadcast_investment_horizon_index function

I have checked that both single horizon and multi-horizon data still works. Todo before merging:

• [x] Confirm all plots still look correct
• [x] Compare this PR against current develop branch to check nothing has changed (other than indexing) - but objective cost and other results should still be the same

Checklist

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

[trevorb1]

@ktehranchi, are you able to take a quick look (and maybe a quick run) to make sure this looks okay, please. Just want to make sure this stays true to your multi-horizon implementation implementation.

Ive run a few different edge cases, and everything seems to work correctly! Comparing against the develop branch, the objective cost continually comes out ~4% different; however the results seem the same (just visually comparing graphs). 4% seems like a lot for simply numerical errors, tho. I can do a better exploration to see where the differences are coming from if you think this is needed.