Updates for packaging and release via PyPI
These changes include:
- modifications to import statements in core python files so that the no3gwt tool can be imported as a package. This is a requirement for PyPi release.
- Addition of a pyproject *.toml file required for Pip distribution and package-building. This file contains metadata and information used by pip to instal the package and solve any dependencies.
- Updated tests to work with any changes.