preprocessing programs updated

allow multiple types of abaqus elem (solid and cohesive) to be used
together.

in fnmclasses.py: added ''index'' attribute to element class

in fnmpreproc_libnee.py: use elem.index to store the global index of the
elem in Abq input file so that its Abq global index is conserved,
facilitating multiple types of abaqus elems to be used.

test3: two brick elems (glb id 1 and 3) and one cohesive elem (glb id 2)