Radioactivedecay

- Adds `decay_high_precision()` method for high numerical precision decay
calculations (adds a new dependency on the SymPy package).
- Code refactoring. Decay dataset format overhauled.
- ReadMe and Docs updates.

- Add support for using `Radionuclide` objects in place of radionuclide strings for `Inventory`
constructor, `add()` and `remove()` methods.
- Add `Radionuclide` `__hash__()` method (needed for above change).
- Add `half_lives()`, `progeny()`, `branching_fractions()` and `decay_modes()` methods to
`Inventory`.
- Add type hinting.
- Add support for calling `len()` on an `Inventory` to find the number of radionuclides it
contains.
- Add support for `==` and `!=` operators with `DecayData`, `Radionuclide` and `Inventory`
instances.
- Add support for `'ps'` (picoseconds) time unit.
- Code refactoring. `Radionuclide` and `Inventory` classes moved into separate files.
- In icrp107_dataset.ipynb, clarify that ICRP-107 does not contain data on spontaneous fission
outcomes.
- ReadMe and Docs updates (mainly updates of the theory and tutorial docpages).

- Add methods to `Radionuclide` class for fetching branching fractions and decay modes.
- Add methods to `DecayData` class for fetching half-lives, branching fractions and decay modes.
- Refactor utility functions into their own .py source file.
- Fix broken links to notebooks.
- ReadMe and Docs updates.
- Add code of conduct and contributing guidelines.

- Restore notebooks folder.
- Add comparison to Radiological Toolbox v3.0.0.
- Add comparison to PyNE v0.7.2.
- Use Black for code formatting.
- ReadMe and Docs updates.
- Add theory section to docs.
- Fix bug in loading custom decay dataset files (allow_pickle=True in np.load()).
- Restore support for Python 3.9.

- Create docs ([https://alexmalins.com/radioactivedecay](https://alexmalins.com/radioactivedecay/)).
- Add NumPy style docstrings to code.
- Add unit tests and code coverage.
- Add option to read in own decay datafiles into a `DecayData` object.
- Add day to year conversion factor as an instance variable of `DecayData` objects.
- Switch day to year conversion used with ICRP 107 dataset to 365.2422 days per year to be
consistent with how ICRP 107 was originally made (this also required updating the
ICRP 107 datafiles created for radioactivedecay).
- Add support for 'ns' (nanosecond) time unit.
- Add operators for multiplication and division of `Inventory` objects (acts on radionuclide
activities).
- Optimize `decay()` method for faster calculations.
- Refactor loading of packaged decay data files in decaydata.py.
- Refactor functions in decayfunctions.py to only take necessary data as arguments.
- Remove Python 3.9 from setup.py as it is not officially released yet.
- Update and improve the ReadMe.

- New support for `Inventory` object arithmetic (addition and subtract of `Inventory` objects).
- New methods for adding, subtracting and removing from `Inventory` objects.
- Added support for more time units ('us' for microseconds, 'ms' for milliseconds, 'ky' for
kiloyears, 'My' for megayears, 'Gy' for gigayears, 'Ty' for terayears and 'Py': for petayears).
Also add support for some other strings for seconds ('sec', 'second', 'seconds'), hours ('hr',
'hour', 'hours'), days ('day', 'days') and years ('yr', 'year', 'years').
- Add new descriptive printable representations for objects.
- Convert `Inventory` radionuclides and activities instance variables to property.
- Associate decay dataset with Radionuclide() objects.
- Improve readme.
- Move Jupyter Notebooks to separate git repository.
(https://github.com/alexmalins/radioactivedecay-notebooks)