major cmake/cuda build framework overhaul

[casperdcl]

• new build framework
  • rely on miutil[cuda], ninst, scikit-build, cmake>=3.18, ninja (all auto-installed via pip!)
  • use new CMAKE_CUDA_ARCHITECTURES policy
  • use new project LANGUAGES specification
  • supports python setup.py clean to cleanup after pip install -e .
• update .gitignore
• basic support for pathlib
• add zenodo test data downloading convenience
  • pytest will recommend running python -m tests to download if not found
• fix prompt when HMUDIR already exported (export HMUDIR not respected #17)
• remove unneeded auxdata items
• basic code tidy (particularly import ordering)