Xdatcar fix

[nkeilbart]

Summary

• Updated Xdatcar class from pymatgen.io.vasp.outputs to update the lattice constants with each new step. Previously, only a single preamble was read in and used for the rest of the structures.
• Created a new XDATCAR test file named XDATCAR_6 to stay consistent with other filenames
• Added a test for Xdatcar that will assure that the lattice constants are different for the first and last image

[coveralls]

Coverage decreased (-0.6%) to 83.032% when pulling 0f65f7b on nkeilbart:xdatcar_fix into 8bb2dd5 on materialsproject:master.

[shyuep]

Thanks.