Fix for #264

[ErichZimmer]

Changes:

• Changed https://github.com/OpenPIV/openpiv-python/blob/2a5b1aef158a22bb3485a0f5ae7511ecc5a69698/openpiv/pyprocess.py#L732 to fix bug with odd sized interrogation windows

• Error message for wrong correlation methods

[alexlib]

Thanks @ErichZimmer for the pull request. the test_correlations.ipynb is clear and demonstrates the solution.