Loading octomap_monitor optional in planning_scene_monitor WorldGeometryMonitor

I'm not using the Octomap mointor or tf, but I always get warnings about these components not working properly. This change provides a bool that defaults to true (enabled) that allows the octomap_monitor not to be loaded. Throughout the planning_scene_monitor there were already a lot of checks to see if the octomap_monitor was already loaded, but I added a few missing ones to be safe.