A better way to create a response function for CSD

This implementation includes the recursive calibration of the response function for CSD, Using an FA threshold is not a very robust method.It is dependent on the dataset (non-informed used subjectivity), and still depends on the diffusion tensor (FA and first eigenvector), which has low accuracy at high b-value. This function recursively calibrates the response function, for more information see Tax et al. NeuroImage 2014.

This implementation is slow only on one line (788) which calls real_sph_harm. I would appreciate any help to optimize this function. Thx!
It gives exactly the same results for me. Indeed, it absolutely has to,
since the formula for sph_harm is exactly the same used in Scipy.

Please try to make a self-contained runnable script --- since I don't
have the code that you are using, it's difficult for me to try to debug it.