remove build powerplants

[trevorb1]

Closes #252

@ktehranchi, can you please confirm that you were not planning to use build_powerplant.py for anything, before I merge this PR. I ran the workflow without build_powerplant and it worked fine

Changes proposed in this Pull Request

Turns out, we dont actually use the build_powerplants.py code anywhere; seems to be legacy from the pypsa-eur code we used as a template. The changes in add_electricity.py over build_powerplants.py are:

• Slight change in update_marginal_costs() function, as described in issue #252
• Change in how marginal cost is calculated for conditional generators (potentially related to PR #249)
  • https://github.com/PyPSA/pypsa-usa/blob/5bc479a6c94de893949410a7ac621dbc70491a2f/workflow/scripts/add_electricity.py#L798
• Addition of attach_wind_and_solar(...) function
• Addition of attach_battery_storage(...) function
• Addition of clean_bus_data(n) function
• Demand is attached under the main function
  • https://github.com/PyPSA/pypsa-usa/blob/5bc479a6c94de893949410a7ac621dbc70491a2f/workflow/scripts/add_electricity.py#L1323
• Capital costs and dynamic prices are updated
  • https://github.com/PyPSA/pypsa-usa/blob/5bc479a6c94de893949410a7ac621dbc70491a2f/workflow/scripts/add_electricity.py#L1381-L1417

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.

[ktehranchi]

yep good to remove.