MODIFIED: Integrators don't remember states now (only DP integrator does
for optimization purposes)
MERGED: Latest engine changes
MODIFIED: Tests modified to use the latest API
ADDED: Kerr metric
REMOVED: unnecessary breaks from Schwarzschild metric code.
FIXED: Added forgotten header
MODIFIED: Particle class now has more virtual functions
ADDED: Draft of Entity class
ADDED: Proper derivative calculation for Entity class.
Moved the engine into a separate directory
Removed readme from branch engine.
FIXED: Some fixes.
ADDED: A simple raytracing program based on the engine and Qt.
Merged branch 'engine' into tests
Adjusted the tests to new file paths.
Merged branch 'tests' into master.
Merged branch 'raytracer' into master
Added new README.