Reduce memory footprint of EAM Hessian calculation
Reduce memory footprint of EAM Hessian calculation

* put calculation of different terms into functions,
  to let unused variables go out of scope and get
  garbage-collected
* reduced footpring of eight term in embedding
  contribution by avoiding calculation of outer
  product

TODO: clean up, do (optional) division by mass outside of
functions, after all non-zero blocks in Hessian have
been computed.

libAtoms/matscipy,libAtoms/matscipy,libAtoms/matscipy,libAtoms/matscipy