Changed forcefields to be class based

[maximillian-dolan]

We aspire to deal with all pull requests in a timely manner. Please be patient.

Ideally pull requests will respect the underlying intrastructure of pylj and agree with PEP8, etc. However, please do not let this put novice programmers off contributing.

[arm61]

Instead of having update_energy and update_force, I would have energy() and force() as methods that return the energy or the force.

Additionally, there is no docstring for the __init__, have a look at how to include these.