Added the Finnis-Sinclair potential from GMIN
Added test for FinSin
Added FinSinCluster in playground

Possible bug in teh potential, or something else is wrong...
Changed defaults for FinSin to Molybdenum and modified the corresponding test.
Added commentary for the Finnis-Sinclair potential.
Merge branch 'master' of https://github.com/pele-python/pele into fin_sin
Merge pull request #79 from dimaslave/fin_sin

add Finnis-Sinclair potential