Radioactivedecay

- Adds plot() method for creating inventory decay graphs using matplotlib.
- New logo in readme and docs.

- Documentation improvements, including improved theory docpage on numerical
computation methods.

- 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.