test/test_rank_filter.py: Added test suite for Python that is equivalent to test/test_rank_filter.cxx to verify that the module works ok and is accurate.
include/rank_filter.hxx: Make sure all window size related variables are unsigned long. Make sure all floating point values are double. Make sure all axis ralated values are unsigned int.
src/rank_filter_py.cxx: Use the same types as the underlying C++ function except allow axis to be negative. This will be adjusted to be within an acceptable range.