Parallel calculations

Support for parallel/async calulations.

Presently, these routines are implemented for VASP. It should be trivial to extend this to any calculator based on external process execution (e.g. abinit or any other which runs blocking shell script to execute the calculation). The support for internal calculators (e.g. ASAP3) should be possible, but is not trivial, and is not planned at the moment. Such in-memory calculators are probably not worth it. The real gain comes from the cluster-run calculations.