[FastRGF] FastRGF doesn't work for small sample and need to fix integration test for FastRGF #92
Description
#Now, sklearn integration tests for FastRGFClassifier and FastRGFClassifier.
FastRGF doesn't work well for small samples, that is reason for test failed.
I doubt inside Fast RGF executable inside.
I inspect Fast RGF by debugger, discretization boundaries are invalid.
At least we should raise understandable error from RGF python if discretization failed.