Fixes #20

Targets PR #18. Kept separate to make reviews easier.

Summary of changes in this pull request

• Adds processing for the World Bank's Global Shipping traffic Density dataset
• Download and use is optional: the unzipped file is 9 GB, so I made use of snakemake's branch functionality to only use it when requested in the config.
• Default is set to match PyPSA-Eur's default of 400(AIS) * 8760(h) * 6(yr) = 2.1e7
  The dataset has cumulative counts of hourly AIS positions (a ship saying "I'm here", per hour) over 6 years.
  It can be seen as roughly "there was an average of 400 ships at this spot per hour between 2015 and 2021".
  The value seems to have been tuned to exclude ports and heavy-traffic routes.

Resampled example for Italy

Example of resulting file for the Netherlands, keeping other defaults the same

Reviewer checklist

• There are no pip dependencies in the module's environment files (workflow/envs/).
• All rules use pathvars (e.g., <results>) in their inputs and outputs.
• The integration test-suite is successful, including:
  • pre-commit.ci tests pass.
  • tests pass for all relevant OS configurations (linux, osx, windows).
• Module documentation is up-to-date, including:
  • INTERFACE.yaml mentions all relevant pathvars and wildcards.
  • README.md describes how to use the module and has the necessary citations.