Fix snakemake dependencies in carbon budget runs

[energyLS]

This PR should resolve issues when running the model with myopic foresight with a carbon budget defined by the 'cb' option. The main issue was that the Snakemake object was not passed to imported functions in prepare_sector_network.py.

The following steps have been done:

1. move the snakemake dependencies of the affected functions to
   compulsory function arguments (in build_energy_totals.py)
2. enter snakemake object values in the function calls
   (build_energy_totals and prepare_sector_network)
3. add the new explicit file dependencies to the Snakefile at rule
   prepare_sector_network

Following other errors have occured and should be resolved:

• The folder created for carbon_budget_distribution.csv included the csv file in the path itself when creating the folders, hence an error occured. Solved in commit
  78d1c92

• The functions build_eurostat_co2 and build_co2_totals from build_energy_totals were missing the 'countries' argument. Solved in commit
  e9e0f3f

• The function build_eurostat_co2 expects the country argument as pd.Index to run the line 25638e3#diff-132a8f9b549899b7eee1fc97ad04baa7c72a349df45041ba0bc1266e58813cacR640 successfully. Solved in commit 25638e3

Remaining questions:

• Should the file dependencies to the Snakefile at rule
  prepare_sector_network added optionally if 'cb' keyword is in
  {sector_opts} wildcard)?
• When running the full snakemake workflow, an error in plot_summary occurs: It tries to read 'results/run_name/csvs/countries.csv' which does not exist. This is no file specified as output in the Snakefile. Where should the 'countries.csv' (originally) be created?

Once the remaining questions are resolved, this PR could be transformed from Draft to ready. Coding improvements are highly welcome. Sorry for the long text ;)

[energyLS]

Reason for error of missing "countries.csv" identified, the file creation was deleted in 1fc1d2a#diff-d838d3f5e300ee6371584a10781b96424387d2186d355c6cf601433ac3e189cbL126

[fneum]

Thanks, @energyLS, and sorry for taking so long to review. Like the compulsory function argument for snakemake. Nice workaround.

Could you add a brief release note to doc/release_notes.rst?

I'd try not to re-introduce the countries.csv. It's just a list of included countries. Maybe one can retrieve it from the PyPSA-Eur config.yaml or n.buses.country.unique().

[energyLS]

Hey @fneum thanks for approving. I will integrate your suggestions (release notes & retrieving countries from config.yaml) in approx. two weeks time.

[lisazeyen]

this can be closed since solved with #256