Aiida-vasp

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This is to be consider the first production release. Backwards compatibility is broken.

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This is a major rewrite to be compatible with AiiDA core beta release. Backwards compatibility is broken.

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**Added**
- ``vasp.base`` workchain which transparently calls through to the 'vasp.vasp' calculation and restarts if appropriate.

- restarting conditions are minimal yet (only submission failed will trigger a retry)

- ``vasp.relax`` workchain which specializes in structure relaxation and optionally iterates until the structure converges.

- 'vasp.calc.immigrant' added in order to support VASP import Aiida-external jobs.

**Changed**
- PotcarData.get_potcars_from_structure() now returns an entry for each ``kind.name`` in the structure, not one per ``kind.symbol``

- PotcarData.get_potcars_dict() no longer fails if there is more than one PotcarData with the same full name in the family

- Minor changes in parser.

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**Changed**
- fixed POSCAR writing (was writing 'direct' followed by cartesian coordinates)

**Removed**
- pruned outdated parts of documentation

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**Changed**
- missing requirement added

- PyPI description and keywords updated

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**Added**
- Allow Structures which do not have sites of same element grouped

- Allow using different potentials for atoms of same element in a structure

- `VaspCalculation`'s ``settings`` input now accepts ``poscar_precision`` key to set maximum precision for coordinates in POSCAR

- This can be indicated in the structure by adding sites with same ``symbol`` but different ``name``.

- ``io.poscar.PoscarIo``, POSCAR writer replacing pymatgen one, to be replaced by parsevasp.

**Changed**
- Use always the same order for species in POSCAR and POTCAR
- POSCAR float precision default changed to 10 decimal places