Radioactivedecay

- Fix bug in decay chain plots which caused overlaps of some radionuclides in complicated chains.
- Document method for installing via conda.
- Improve clarity of matplotlib imports.
- Correct a DecayData attribute docstring entry.

- Improve readme and fix typo.
- Add MANIFEST.in so LICENSE and markdown files get packaged.
- Update year in LICENSE file.

- Allow specification of `sig_fig` as a parameter to the `decay_high_precision()` method.
- Improve code with respect to pylint and mypy checks.
- Add discussion on the radioactive progeny that are not in the ICRP-107 data to the limitations
section of the docs and to the Jupyter notebook analyzing the ICRP-107 data.
- Update other Jupyter notebooks for this release.
- Theory docpage typo fix.

- Fix bugs upon supplying Matplotlib Figure or Axes objects to `Inventory` and `Radionuclide`
`plot()` methods.
- Docs and ReadMe updates.
- Changelog typo fixes.

- Add `plot()` method to `Radionuclide` class for creating decay chain diagrams. This adds a
dependency on the NetworkX library.
- Changes to `plot()` method of `Inventory` class: fix for bug which ignored the user setting of
`npoints` when using high precision decay mode, docstring fixes, no longer call `plt.show()`.
- Mistakes in comments of unittest python files.

- Add `Inventory.half_life()` method as spelling variation of `Inventory.half_lives()`
- ReadMe: simplify. Use `'readable'` for inv_t1 half-lives example.