Revert "Added a velocity field to the Ball class."

This reverts commit ecf2c5c2aaf7381d52e825edaf837744cc13dcca.
Merge branch 'geometry' into develop
Refactored a bit.

Moved the Ball2D class to the geometry package, seemed a bit silly to
have a sub package with one class.
Merge branch 'geometry' into develop
Added the Ball class to the movables package.

The difference between a movable and a normal geometric figure is that
a movable has position and velocity, whereas a normal figure only has a
position.
Merge branch 'movables' into develop
Added the Box and BoxSize classes.

The Box class will basically act as a box filled with balls.
Merge branch 'box' into develop