Added options to superimpose to merge_structures.py
Added options to superimpose to merge_structures.py

Pretty self explanatory. See the help for merge_structure.py on how to use
the superposition tool.

Not sure if the code is the most efficient possible, but I got it working
quickly.

arosale4/q2mm,ericchansen/q2mm,arosale4/q2mm,Q2MM/q2mm,ericchansen/q2mm,Q2MM/q2mm