Implement short-range order observer

Description

Short range order is nice.
To your paper it adds some spice.
Sometimes it looks good, sometimes it looks bad.
One time it looked like a bag of brambles.
But then you just need to put in more samples.

Background

A widely use short-range order (SRO) parameter is due to Cowley (Phys. Rev. 77, 669 (1950)). It is defined as follows (taken from this paper for simplicity)

\alpha^{(k)}_\text{B} = 1 - \frac{Z_\text{A}^{(k)}}{Z_\text{tot}^{(k)} (1-c_\text{B})}.

Here, k denotes the neighbor shell, Z_\text{A} is the number of neighbors of type \text{A} in said shell, and c_\text{B} is the concentration of \text{B} in the sample.

The SRO observer can thus be implemented rather easily using the ClusterCounts class.

Demo

• all tests pass
• new functionality fully tested
• docstrings complete and included in user guide

edit (PE, 2018/12/04): apples -> brambles; added background, demo