Radioactivedecay

- Fix bug where high-precision (SymPy) default dataset had incorrect half-lives for Th-232, Sm-147
and Rb-87 (82). All users performing `InventoryHP` decay calculations for chains containing these
radionuclide are encouraged to upgrade without delay. Previous radioactivedecay versions gave
incorrect results (note normal precision `Inventory` decays were unaffected).

- Code improvement to support metastable states up to 6th state (76).
- Added labels for more decay modes (including heavy cluster decay modes) to decay chain plots code
(77).
- Improve parsing of nuclide strings (78). Note: you must use the correct capitalization of the
metastable state and the element chars when using mass number first format, e.g. use '130nI' to get
I-130n, not '130ni', '130Ni' or '130NI' as these will be mistaken as Nickel (Ni). Capitalization
does not matter if using element first formats, e.g. 'I-130N' or 'I130N' will be correctly parsed
to I-130n.

- Added more stable products to the default ICRP-107 dataset (75).

- 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).