Added a missing multi element POTCAR file

[MichaelWolloch]

and reverted the file loading change in commit e3bcad7

Summary

• Fix 1: Fixed a unit test that failed due to a missing file by actually committing the new file. The unit test was already fixed by @janosh in e3bcad7, by taking another POTCAR, but this kind of defeated the purpose of the test which was to check multi-element POTCARs.

[janosh]

@MichaelWolloch Thanks for the fix! 👍

[MichaelWolloch]

Thank you @janosh for finding the missing file!

[coveralls]

Coverage: 78.019% (-0.7%) from 78.738% when pulling fd99292 on MichaelWolloch:master into 0d2062a on materialsproject:master.