src folder and pyproject packaging

[shimwell]

Just seeing if the package could be updated to follow the latest python packaging recommendations https://packaging.python.org/en/latest/tutorials/packaging-projects/

• use of a src folder - helps avoid local import using folder name instead of installed package
• use of pyproject.toml instead of depreciated setup.py
• use of setuptools_scm to get version number

3 benefits of src folder layout are detailed at the bottom of this page

[shimwell]

this PR has some major merge conflicts since #110 was merged so I shall close it for now. If there is any interest in moving to a src folder layout for the package, updating the setup to use pyproject.toml or automatic the versioning with setuptools_scm give me a nudge and I can do this PR or aspects of it again