% update gradients from an initial gradient set, such that the updated gradients are evenly distributed.
% INPUT (required)
% gradCellInitial : N_k x 3 gradient matrices, where each row is a point in sphere. k=1,2,...,K
% param.w : weight for single shell term. 0<w<1. 0.5 is the default value.
% INPUT (optional)
% param.solver : 'nlopt' (default): use sqp solver (SLSQP) in NLOPT.
% 'matlab': use fmincon (sqp solver).
% param.cartesian : true (default): use cartesian coordinate (with unit equality constraint);
% false: use spherical coordinate (no unit equality constraint).
% param.localCon : 0: do not use local constraint
% 1 (default): use local constraint for the distance between samples in results and samples in initialization to reduce the inequality constraint.
% It is faster, and with an appropriate localConAngle, it can obtain the same results as it is set as false.
% param.localConAngle : It is the maximal angular change between samples in results and samples in initialization.
% It is used when param.localCon is true. It is automatically set based on the upper bounds, if it is not given and param.localCon=true.
% If localConAngle>pi/2, then the result is the same as the results without localCon.
% gradCell : Kx1 cell, each element is a gradient matrix.
% "Novel single and multiple shell uniform sampling schemes for diffusion MRI using spherical codes",
% Jian Cheng, Dinggang Shen, Pew-Thian Yap, Peter J. Basser, MICCAI 2015.
% Copyright (c) 2014, Jian Cheng <firstname.lastname@example.org>