Configure doctests

[r-akemii]

This PR finishes the doctest setup for the existing code in docstrings within AequilibraE documentation. Due to testing limitations, some tests were removed.

In this PR we:

• Modified docstrings in the following files:
  • distribution/gravity_application
  • distribution/gravity_calibration
  • distribution/ipf
  • matrix/aequilibrae_data
  • matrix/aequilibrae_matrix
  • paths/results/path_results
  • paths/results/skim_results
  • paths/assignment_paths
  • paths/graph
  • paths/network_skimming
  • paths/sub_area
  • paths/traffic_assignment
  • paths/traffic_class
  • paths/vdf
  • project/network/link_types
  • project/network/link
  • project/network/links
  • project/network/modes
  • project/network/network
  • project/network/node
  • project/network/nodes
  • project/network/period
  • project/network/periods
  • project/about
  • project/field_editor
  • project/project
  • project/zoning
  • transit/transit
  • log
  • parameters
• Updated the .github/workflows/documentation.yml to run doctest
• Added variables to doctest_namespace in conftest.py to make the code less cluttered
• Removed unused doctest projects in docs/create_docs/data.py
• Updated the doctests in *.rst files with docstest_namespace variables
• Created a traffic_assignment_procedure file to keep the tests in assignment_mechanics independent

[r-akemii]

@pedrocamargo before opening this PR, I'd like to check with you if it's ok if we don't test the strings in docs/source/useful_information/validation_benchmarking/traffic_assignment.rst. In this test, we use TNPT files stored locally, so it's a bit more complicated to write tests with doctest.

If you want, we can also add tests to docs/source/modeling_with_aequilibrae/static_traffic_assignment/assignment_mechanics.rst. I'll probably have to create a project folder for it, and re-write part of the code blocks.