BF: removed 1/(4*pi) factor for fod computation

This fixes the problem: dwis look good, fODF can be computed from them and the speed is fine. 
I guess the problem is the same (1/(4 * np.pi)) in compute_fod_sh.

The 1/(4 * np.pi) makes the diffusion value very low compare to the b0 value and i guess this makes the fODF computation (dipy) fails. 

What should be done?