Defined destructors for SlipPlane and SlipSystem
The SlipPlane destructor needs to be tested for segmentation faults.
Added explicit definitions to the default constructor for SlipPlane
Deleting dislocations, sources and defects separately and only once.
Replaced SlipPlane::extremeties[2] with SlipPlane::extremities as a
pointer to Defect. This will allow us to delete it later directly using
the pointer to the instance.
Merge branch 'develop' into uniqueID