Added some basic geometric models.

Added classes for working with points and vectors.
Refactored a bit.

The “Pair” classes have been moved to the package
ballcollider.controllers, since they are more controllers than models.
Added ball class to ballcollider.models.geometry.shapes.

Added ball class to ballcollider.models.geometry.shapes.
Added a velocity field to the Ball class.
Merge branch 'geometry' into develop