Core & API Redesign

Where:

https://github.com/janpipek/physt/tree/v05

Requirements:

- axis type with abstraction for bin labels etc.
- support for units?
- at least partial conceptual compatibility with https://github.com/HDembinski/histogram
- seamless usability for pandas, xarray, np.ndarray

Principles:

- orthogonality wherever possible

What has to be done: