Fixes for Cone Visualisation

Proposed fixes for cone visualisation as per issue: https://github.com/PointCloudLibrary/pcl/issues/866#issuecomment-53691601

--- shapes.h ---
Correction to documentation

--- shapes.cpp ---
The change to cone->SetCenter() is a bug fix.

The change to cone->SetHeight() is a proposed 'improvement' allowing the cone height to be set using the magnitude of the direction vector.