Include guards

[acxz]

This PR adds missing include guards via #pragma once to various headers which are missing them.

[acxz]

The header files under python/ and wrap/ still have missing guards. I wasn't sure whether to add them or not. Let me know if we should and I'll add them rq.

One more file gtsam/base/debug.h has also not been baptized. This is because the multiple include is required for switching #defines in the test case check_base_program.

[varunagrawal]

python headers don't have guards since they serve a difference purpose. Though I guess adding guards would help, we would have to ensure the wrapping process doesn't break.

If you want to include guards in wrap, then do so here.