Modified diffuse reflection in pathtracing

Use average color intensity instead of maximum.
Bugfix in pathtrace

Multiplication with 1/PI instead of PI caused random rays to become not
so random.
Octree optimalization. Do not check intersection if a node does not contain any triangles.
Add the possibility to read free form meshes from config file.
Rename test files and add new tests.
Add converter from obj format to our own mesh format.
Format test output.
Support multiple lights in ray tracer.
Introduce Intersection class.