[j-brendel]

Central base directory (data_dir) for relative input/output paths

Problem

All paths in a SimBA config (schedule_path, electrified_stations_path, output_path, ...) and
the csv_file entries inside electrified_stations.json are resolved relative to the current
working directory, not relative to the config file.

As a result, scenario data has to live inside the SimBA repository (e.g. data/private_examples/),
or every file has to contain absolute paths. Moving a scenario folder, e.g. confidential project
data to a separate (cloud) drive outside the repository, means editing paths in many files.
Absolute paths are machine-specific, so configs cannot be shared between colleagues.

Proposed solution

Add a new option data_dir that serves as a central base directory for relative paths:

data_dir = .                      # relative to this config file (or absolute)
schedule_path = trips.csv
electrified_stations_path = electrified_stations.json
output_path = results

• data_dir can be set in the config, via --data-dir, or via the environment variable
  SIMBA_DATA_DIR. With the environment variable, no machine-specific path has to be stored in
  any file.
• A relative data_dir is resolved relative to the config file.
• What is resolved: explicitly set relative path arguments, the csv_file entries in electrified
  stations (fixed_load, local_generation, price_csv, ...), and decision_tree_path in the
  optimizer config.
• What is not changed: absolute paths, and path arguments that still have their default value
  (e.g. vehicle_types_path). Those stay relative to the working directory.
• Without data_dir: the behaviour is unchanged (fully backwards compatible).

Implementation (branch feature/data_dir)

• simba/util.py: new argument --data-dir, helpers resolve_path() and resolve_data_dir(),
  called in get_args() after deprecated arguments are replaced.
• simba/data_container.py: resolve_csv_files() is applied to the electrified stations in
  add_stations_from_json(), so SimBA and SpiceEV receive absolute paths.
• simba/simulate.py: resolves decision_tree_path of the optimizer config.
• Tests in tests/test_util.py and tests/test_data_container.py. Docs in
  simulation_parameters.rst and extensive.cfg.