versioning / packaging

[KangchengHou]

1. replace setup.py with pyproject.toml. setup.py can still be useful for editable installment (but as this packge get mature, we may not that anymore).
2. add versioning to each update.
3. update to pip

Timeline:

1. Have a releasable version and upload to pip (this can be done after going through the whole package) (pyproject.toml update happens here.)

[KangchengHou]

there is a admix/version.py tracking the version. Because we have customized package of pgenlib it is hard to release to pip. The current recommendation is to use git clone and pip install -e .