bugfix: fix classname in doxygen docu.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
refactoring : rename file ITask.h to AbstractTask.h because of the corresponding class.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
- update : add a pointer to the task scheduler instance to the task group. this will be used to schedue depending tasks to pending threads.
- bugfix : fix a assert in ce_firsttriangle. the attribute mask was missing.
- bugfix : handle meshes without an assigned material in the class GeoNodeRenderCallback. now a default material will be created.
- bugfix : fix invalid include.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
- fetature : add unittest for the class TaskGroup.
- update   : add notification wen resource loading is finished.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>