Added 'monoatomic' to title & descriptions

I've clarified that the 'monovacancy-formation-energy-cubic-diamond' and 'monovacancy-formation-free-energy-cubic-diamond' properties are for monoatomic diamond in their titles and descriptions (there are multispecies cubic diamond lattices, e.g. SiGe).  Let me know if I should add 'monoatomic' to the titles.

I've also changed the key name 'cohesive-energy' to 'cohesive-potential-energy' in the 'cohesive-energy-relation-cubic-crystal' PD.
-Dan