Fixing assets that had problems loading (#3528)

- Correctly remove interpolating render program to prevent invalid shader access
 - Fix Apollo 11 location
 - Fix Apollo 17 location
 - Replace errors with warnings for Gala and Rime instruments for Juice
 - Correctly rely on the solarsystembarycenter asset in the Voyager/Pioneer asset
 - Use correct identifier for the Jupiter barycenter
 - NASA Treks assets
 - Slidedeck
 - Fix unloading of missions
 - No longer crash when loading and unloading image sequence assets

include/openspace/mission/mission.h

@@ -77,6 +77,13 @@ public:
      */
     const std::string& name() const;
 
+    /**
+     * Returns the unique identifier of the MissionPhase.
+     *
+     * \return The unique identifier of the MissionPhase
+     */
+    const std::string& identifier() const;
+
     /**
      * Returns the TimeRange of the MissionPhase.
      *
@@ -168,6 +175,8 @@ protected:
 
     /// The name of the MissionPhase
     std::string _name;
+    /// The identifier of the MissionPhase
+    std::string _identifier;
     /// The description of the MissionPhase
     std::string _description;
     /// The range in time that is covered by this MissionPhase