Added LDGM as a submodule.

LDGM should be now self-contained, without any dependency on UltraGrid.

This is a workaround over NVIDIA driver bug that causes Linux kernel bug
when there are 2 graphic cards in system and context is initialized from
within other thread than is used from.

TODO: replace gettimeofday() calls (now commented out) with
std::chrono::steady_clock (or similar).

+ added API for universal module