Feature/gaia mission merge (#766)

* Add Adam Aslegård's module that renders Gaia mission stars
 * Add module for Gaia mission
 * Add CCfits and cfitsio submodules and the implemented fitsfilereader module from the OpenSpace-sun-earth-event branch
 * Add a TaskRunner to read from a fits file

include/openspace/util/distanceconversion.h

@@ -289,6 +289,8 @@ constexpr double convertDistance(double meters, DistanceUnit requestedUnit) {
     }
 }
 
+float convertMasPerYearToMeterPerSecond(float masPerYear, float parallax);
+
 } // namespace openspace
 
 #endif // __OPENSPACE_CORE___DISTANCECONVERSION___H__