Added the visitor API, calculate center/radius

Added basic visitor pattern, implement a simple visitor to calculate the
center/radius of the scene for rendering.

Change-Id: I211bda04030157f160e1c07fbbd71e6d02c05380
Clean up the scene class a little

Remove variables that are no longer used, set a minimum radius so that
drawing continues to work well for an empty scene.

Change-Id: I14a7d8b77fd9a2916642ec4ad1657f9a0f6f4577
Make the geometry visitor accumulate geometry

Attempt to accumulate geometry, this has received limited testing (and
more should be added to ensure the averaging is sensible) but it seems
to work with a large range of the molecules I tested it with.

Change-Id: Ie065a9710b314f711606c9a8b292ea21abf49cfd
Merge topic 'rendering-visitor' into master

519b6bf7 Make the geometry visitor accumulate geometry
3dab40cd Clean up the scene class a little
141712bb Added the visitor API, calculate center/radius