Sanitize unit test and increase # significant digits PC gradients#721

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pultar wants to merge 2 commits intogrimme-lab:mainfrom pultar:precision-pcem

+63-68

Commits on Nov 17, 2022

Increased significant digits for point charge gradients to 12
pultar
committed
Second unit test in C API is water tetramers: https://xtb-docs.readthedocs.io/en/latest/pcem.html . Be more consistent with Fortran unit tests
pultar
committed