Removed unused code.
Refactoring of DiagramCreator code.  Added comment for AngleIterator. Renamed AngleIterator::next_angle as AngleIterator::nextAngle. Added a new interface called GuideSizeStrategy and extracted some code from DiagramCreator to write a GuideSizeStrategy implementation (for the moment, called JeansGuideSizeStrategy).  Give a DiagramCreator one of these implementations in its constructor (in future, we could implement other strategies and add a set-method for this strategy).