Build issues with current versions of libsgp4

[anoved]

Starting from a clean clone, the documented build process now fails because of changes introduced to the SGP4 library (different files, build process, method names). The Makefile-sgp4 distributed with gtg no longer works. Update for compatibility with latest SGP4 library and, license permitting, roll it into this repository to ensure reliable builds.

[anoved]

Deleting references to Julian.cpp/Julian.h and changing CoordTopographic to CoordTopocentric is sufficient to allow make lib to succeed with Makefile-sgp4. However, gtg's source code includes Julian.h as well. At minimum, I think references to Julian should be changed to Datetime. But again, now it would be best to rely on the lib's build process if possible.

At either rate, some work is needed for API compatibility with the current revision of SGP4++.