PREP_DIR instead of hard-coded "Player-Data/" when suitable.

[kekomal]

Inside C++ code there was a few hard-coded references to "Player-Data/" directory when referring to default input folder. These strings may lead to inconsistencies if modifying -DPREP_DIR argument when compiling.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[mkskeller]

Would you mind the signing the CLA despite the small change?