Radioactivedecay

- 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.

- Improve `'readable'` half-life strings for radionuclides with half-lives less than 1 s or
greater than 1000 years.
- Switch from reporting microseconds as `'us'` to `'μs'`.
- Docs improvements: required packages and acknowledgements.
- ReadMe: Matplotlib dependency is used for plotting.
- Changelog typo fixes.

- Supply `'readable'` as time unit parameter to half-life methods to get strings with the
half-lives in human-readable units.
- Add support for unicode `'μs'` and for `'By'` (billion year) time units.
- Changed the `Inventory.plot()` graphs so that the decay curves are ordered from highest to lowest
radionuclides in the decay chains by default. Alphabetical ordering can be specified by supplying
the new `order=='alphabetical'` parameter to the method.
- `DecayData.half_life()` method docstring typo fix.

- Fixed version numbers.
- Updated logo.
- Use logo hosted on GitHub in ReadMe.