ENH: add version number to Vasp calculator and vasp hessian paraser

Lukasz Mentelrequested to merge
lmmentel/ase:vasp-enh into master

Vasp calculator now accepts an integer vasp_major_ver argument that is propagated to the io.vasp.write_vasp to write the POSCAR in the correct format. A hessian matrix can now be parsed from the OUTCAR as a numpy array.

Also style improvements in the io.vasp.write_vasp function.

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