Unit test build shapes and minor changes to test_base_network.py

[finozzifa]

Closes # (if applicable).

Changes proposed in this Pull Request

The pull request proposes the following unit tests:

test/test_build_shapes.py:

• test_simplify_polys
• test_countries

conftest.py:

• I added the italy_shape fixture. It replaces the test/test_data/italy_shape.geojson file (which in fact has been deleted).

test/test_base_network.py:

• I replaced italy_shape.geojson with the fixture italy_shape

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.