insure int type for math.factorial

[ll01]

https://bugs.python.org/issue37315 Accepting floats for math.factorial was removed. since d is either even or manipulated to be even, it should be safe to convert to an integer. https://stackoverflow.com/questions/21753841/factorial-in-numpy-and-scipy math.factorial and np.math.factorial is the same thing

[paulknysh]

Hi, sorry, I somehow missed this PR earlier. I also just did major code refactoring - feel free to re-open this to be in sync with updated master brach.