Added vector clear functions to SlipPlane class.
Check for dislocations and dislocation sources before creating the
respective vectors.
If this check is not performed, we end up with vectors containing 1 item
even when there are none present due to the allocation at the time of
declaration. Therefore, this check clears the allocated memory and
leaves the vector container with size 0.
SlipPlane::defects is cleared and rebuilt in the function updateDefects.