code: public CollisionGroup getCollisionGroup(String name)
      {
          if (groups.containsKey(name))
          {
              return groups.get(name);
          }
          throw new LionEngineException(ERROR_FORMULA + name);
      }
docs: Get the collision group from its name.
  
  @param name The collision group name.
  @return The supported collision group reference.
  @throws LionEngineException If group not found.