CometMgr update

To let a user write scripts that control the comet's appearance (tail lengths, coma sizes, etc.), a CometMgr (StelModule) needs to be added for the Stellarium scripting engine.

Initially, the idea was the derive CometMgr from SolarSystem, and separate Comets with the rest of the SolarSystem bodies. But I found that this duplicated and was just messy and unnecessary. So instead, CometMgr will be a friend class of SolarSystem so that it has access to Stellarium's active list of comets, and the current comet that has been selected. Using CometMgr, we change the member variables of a Comet object.

Update July 25: CometMgr done.