Refactored Snakemake tests into pytest

• usable python bindings in build dir venv (without using wheel-build via pip install .)

  # Configure with OGS_USE_PIP=ON
  cd build-dir
  source .envrc # `Invoke-Expression .envrc.ps1` (on Windows Powershell)
  ninja
  pytest ../path/to/source-dir

• some pytest unit tests via ctest (ctest -R pytest)
• updated ogstools, can be used in notebooks, see SimpleMechanics notebook for an example
• refactored Snakemake tests into pytest
• removed snakemake dependency

TODO:

• Fix wheel build
• Later: Update to next ogstools release which will contain ogs6py, remove ogs6py dependency.

---

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?