PCLVisualizer fixes

[VictorLamoine]

The size of the model coefficients must be checked to avoid undefined behaviors.
I updated the documentation of addCone (which was false) and addCube; I redirect to shapes.cpp which has code examples.

The doc link have been tested.

[taketwo]

Would it make sense to print a PCL_WARN? I doubt that people routinely check return value of addXXX() functions. Seeing a friendly message in console when they supply wrong coefficients will help them to localize the problem faster. (It's not obvious what is wrong when you just don't see anything it the visualizer after adding a shape.)

[VictorLamoine]

Agreed!