Add thread safety for model manipulation
Merge branch 'thread_safety'

Conflicts:
	lib/SingleEyeFitter.cpp
	lib/singleeyefitter/singleeyefitter.h
Re-run cmake after installing external projects to set up library paths correctly
Forward some cmake cache variables to pupil tracker
This allows the pupil tracker to use non-standard library paths
Rename PUPILTRACKER_INSTALL_DIR to the more common *_ROOT_DIR