Merge pull request #13 from fvanderlinden/master

Forces visualizers and extra forces
 * added inner Modelica.Mechanics.MultiBody.World as required by Modelica.Mechanics.MultiBody.Visualizers.Advanced.Arrow and Modelica.Mechanics.MultiBody.Visualizers.Advanced.DoubleArrow
 * The state variable PlanarMechanics.Joints.Prismatic.s is of type Modelica.SIunits.Distance which means that the minimum length is zero. This is not suitable for simulation which negative elongations. See also [Mo#446](https://trac.modelica.org/Modelica/ticket/446) where there was the same problem.
 * Did some cosmetics.
Merge pull request #14 from tbeu/master

Issues with SI.Distance and MB.World