Radioactivedecay

- Default `ymin` for Inventory `plot()` method is now 0.95x the minimum quantity that occurs over
the decay period when `yscale='log'` (70 & 72).
- Improved nuclide string parsing: robust to nuclide strings containing whitespaces and some
incorrect capitalizations of the element / metastable state chars (65 & 72).
- Fix `load_dataset()` docstring not showing in decaydata API docpage (71).

- Fix incorrect parsing of SymPy version string (67, 68). This bug meant radioactivedecay import
failed if using SymPy >=1.10. The fix makes Setuptools an explicit dependency.

- Code refactoring: reduce coupling between modules by refactoring Converter, Nuclide & Inventory
classes to not store duplicate data, but to receive data when needed via their API calls. DecayData objects
no longer store Converter objects. Continuing to reduce pylint/mypy errors/warnings, however will
need to wait to bump requirement to Python 3.7+ to improve type hinting of NumPy/SciPy arrays
(requires NumPy 1.20+, holding off for now to maintain Python 3.6 support) (64, 66).
- Improve API documentation table of contents & README fix (66).
- Better unit tests for DecayData class and error handling is instance does not contain SymPy data
(66).
- Move dataset creation and comparison notebooks to separate repos within the radioactivedecay org
on GitHub (63).
- Tweak GitHub Issue & PR markdown templates (62).

- Fix some code bugs and make other improvements to the ReadMe and the Docs (57, 58 & 59).
- Reduce number of mypy errors, use Python f-strings everywhere (60).
- Fix the URL for the codecov badge in the ReadMe (56 & 60).
- Introduce GitHub Actions for CI/CD. Add Issue & Pull Request templates (61).

- Fix old references to `Radionuclide` class in Readme, docs and docstrings (54 & 55). This class
was renamed to `Nuclide` from release v0.4.0.
- Other small rewrites to docs.

- Projected transferred into radioactivedecay organization on GitHub: updated code & docs
accordingly.
- Moved hosting of docs to GitHub Pages (https://radioactivedecay.github.io).
- Opened forum for project at https://github.com/radioactivedecay/radioactivedecay/discussions
(uses GitHub Discussions).
- Only store radioactivedecay version number in radioactivedecay/__init__.py file. Read this file
to obtain version number in setup.py and docs/source/conf.py.