Sum-of-squared constrained CSD

[rutgerfick]

Following the work of Dr. Dela Haije, it is of interest to see if "true" positivity constrained CSD has an influence on microstructure prediction.

This issue is to include the sum-of-squared constrained CSD optimization in dmipy as a generalized optimizer.

Haije, Tom Dela, Evren Özarslan, and Aasa Feragen. "Enforcing necessary non-negativity constraints for common diffusion MRI models using sum of squares programming." NeuroImage 209 (2020): 116405.