Alns

Fixes issue 20, where `plot_operator_counts` did not scale its x-axis with the actual data to be plotted.

Changelog:

- Call-back mix-in class with options for acting on events

- Operator decay parameters in [0, 1], rather than (0, 1).

- Optional title argument for `Result.plot_objective`

- More flexible `Result.plot_operator_counts`

Changelog:
- Test and support for Python 3.8

This release adds support for operator selection diagnostics on the ALNS `Result` class.

Changelog:
- Merged in 16, which closes 9.
- Added plot method for operator counts, as `result.plot_operator_counts()`
- Added (optional) operator naming, via `alns.add_<operator type>_operator(operator[, name="name"])`

- Update Readme
- Simplify CSP example