#171 Delete the difference between the elements on marks 1 and 2 in HyperMesh tcl

#171 Delete the difference between the elements on marks 1 and 2 in HyperMesh tcl.

*markdifference determines the difference between two marks and stores the results in the first mark.

*createmark elems 1 1 2 3 4
*createmark elems 2 4 5 6 7
*markdifference elems 1 elems 2
*deletemark elems 1

This will delete elements 1, 2, and 3.