Fix creation of POTCAR.stripped.stripped etc files

Merged David Sikter requested to merge 979-creation-of-potcar-stripped-stripped-etc-files into develop

Closes #979 (closed). Makes sure that we don't create a stripped version of a POTCAR file that has already been stripped, and that we exclude files named POTCAR*.stripped.stripped when packaging the public files, so the redundant files will disappear when reprocessing.

Edited by David Sikter

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