Impactlab-tools

- Drop unused ``h5netcdf`` dependency. Make ``matplotlib``, ``geopandas`` optional dependencies.

- Add new extras for optional dependencies: ``impactlab-tools[viz]``, ``impactlab-tools[complete]``.

- Minor code cleanup, style update.

- Update ruff lint section format in pyproject.toml.

- Minor update to docstr examples to match ``xarray`` > 2023.12.0.

- Drop Python 2 support. Requires Python >= v3.7.

- General documentation build update. Update to readthedocs v2 configuration.

- Update package dependencies. Require previously unlisted dependencies: ``h5netcdf``, ``geopandas``, ``matplotlib``, ``numpy``, ``pandas``, ``scipy``, ``xarray``.

- Fix error from bad coordinate assignment in ``impactlab_tools.gcp.reindex.hierid_to_shapenum()``.

- Remove dependence on ``toolz`` package.

- Quick and dirty code style cleanup.

- Replace ``flake8`` with ``ruff`` for code quality checks.

- Consolidate requirements files into requirements.txt, purely used for Github Actions CI.

- Update and refactor into ``src/`` layout with ``pyproject.toml``. Radically simplify the 'meta' of building and maintaining the package.

- Drop ``tox``.

- Add LICENSE file to repo.

- Remove Travis-CI configurations. Use Github Actions for CI (:issue:`468`)