rewritten hs-wca potential

the hs-wca potential has been rewritten such that no square roots need to be computed, these changes should improve performance. Note that the functional form of the potential is marginally different now.
removed extra level of indirection from operator[], now returning a copy. For doubles copy is cheap
remove using:: and added coordinates file for test
bug fix to hessian and fixed tests
Merge branch 'master' of https://github.com/pele-python/pele into new_hswca
Merge branch 'master' of https://github.com/pele-python/pele into new_hswca
Merge branch 'master' of https://github.com/pele-python/pele into new_hswca
revert change to Array::operator[]
Merge pull request #51 from smcantab/new_hswca

Fast hswca now implemented