ENH: Added site occupancy factor observer together with a unittest.

Description

This merge request adds a SiteOccupancyFactor class, which is an Observer that can be used to compile information regarding site occupancy factors, as well as a unittest script.

Comments

Out of the three functions for obtaining information from DataContainer objects that have been defined, only the first, which extracts data for all trajectories, is actually required. The other two, which provide the average values and standard deviations, are not necessary but have been included since the DataContainer class have such functions.

Though the unittest script works, it might not be optimally written and should be considered quite carefully so as to ensure that it encompasses all possible issues.

Summary of changes

The following files have been added:

• mchammer/observers/site_occupancy_factor_observer.py
• tests/unittest/test_mchammer/test_site_occupancy_factor_observer.py

Closes #166 (closed)