Add pytest unit/integration markers and test-running docs

[SrijaVuppula]

Summary

This PR adds clearer pytest-based test separation and short contributor docs for running tests by marker.

Changes

• add unit and integration markers to pytest.ini
• mark:
  • tests/test_utils.py
  • tests/test_weather.py
    as unit
• mark:
  • tests/test_genetic.py
  • tests/test_direct_power_method.py
  • tests/test_maripower_tanker.py
    as integration
• add a Running tests section to CONTRIBUTING.md

Verification

Ran unit tests:

./.venv/bin/pytest -m "unit" tests/test_utils.py tests/test_weather.py -q
# Result: 31 passed

Ran integration tests:

./.venv/bin/pytest -m "integration and not manual and not maripower" tests/test_genetic.py tests/test_direct_power_method.py tests/test_maripower_tanker.py -q
# Result:
- 15 passed
- 1 skipped
- 14 deselected

Notes

Manual tests currently depend on a local LaTeX installation and were not part of this PR.