Check if badly ordered StructureData is translated into POSCAR correctly

[DropD]

POSCAR does not allow for example [Ti, O, Ti, O], instead the species have to be together as i [Ti, Ti, O, O].

[DropD]

Findings:

1. unsorted structure data is by default translated to unsorted pmg / ase structure
2. unsorted pmg / ase structures are written to unsorted POSCAR
3. unsorted pmg structures can be sorted in place using the sort() method

The following snippet will lead to well formed POSCAR:

from pymatgen.io.vasp.inputs import Poscar
from aiida.orm import DataFactory

unsorted_st = DataFactory('structure')()
unsorted_st.append_atom(..., symbol='As')
unsorted_st.append_atom(..., symbol='In')
unsorted_st.append_atom(..., symbol='As')
unsorted_st.append_atom(..., symbol='In')

pmg_st = unsorted_st.get_pymatgen()
pmg_st.sort()
pmg_poscar = Poscar(pmg_st)
print pmg_poscar.get_string()

>>>
In2 As2
1.0
1.000000 0.000000 0.000000
0.000000 1.000000 0.000000
0.000000 0.000000 1.000000
In As
2 2
direct
0.000000 0.500000 0.000000 In
0.500000 0.000000 0.000000 In
0.000000 0.000000 0.000000 As
0.000000 0.000000 0.500000 As