Reworked how SPICE kernels are loaded

- SpiceEphemeris can provide a list of kernels that are loaded when created
- Time and Leapsecond kernels are provided in the openspace.cfg file

include/openspace/util/constants.h

@@ -35,7 +35,8 @@ namespace configurationmanager {
     const std::string keyConfigSgct = "SGCTConfig";
     const std::string keyConfigScene = "Scene";
     const std::string keyStartupScript = "StartupScripts";
-	const std::string keyConfigTimekernel = "SpiceTimeKernel";
+	const std::string keySpiceTimeKernel = "SpiceKernel.Time";
+	const std::string keySpiceLeapsecondKernel = "SpiceKernel.LeapSecond";
 } // namespace configurationmanager
 
 namespace scenegraph {
@@ -100,6 +101,7 @@ namespace staticephemeris {
 namespace spiceephemeris {
     const std::string keyBody = "Body";
     const std::string keyOrigin = "Observer";
+	const std::string keyKernels = "Kernels";
 } // namespace spiceephemeris
 
 }  // namespace constants

include/openspace/util/spicemanager.h

@@ -57,7 +57,7 @@ public:
 	 * \param kernelId  unique integer ID for the loaded kernel
 	 * \return          loaded kernels/metakernels unique integer id
 	 */
-	int loadKernel(const std::string& fullPath, 
+	int loadKernel(std::string fullPath, 
 		           const std::string& shorthand);
 
 	/**

include/openspace/util/time.h

@@ -52,14 +52,11 @@ namespace openspace {
 class Time {
 public:
 	/**
-	 * Initializes the Time singleton and loads an LSK spice kernel with the provided
-	 * name.
-	 * \param lskKernel The name of the kernel that should be loaded during the
-	 * initialization. If the parameter is empty, no kernel will be loaded
+	 * Initializes the Time singleton.
 	 * \return <code>true</code> if the initialization succeeded, <code>false</code>
 	 * otherwise
 	 */
-	static bool initialize(const std::string& lskKernel = "");
+	static bool initialize();
 
 	/**
 	 * Deinitializes the Time singleton. This method will not unload the kernel that was