% Calculate the covering radius of the given samples.
% radius = CoveringRadius(grad)
% radius = CoveringRadius(grad, ind)
% grad : Nx3 matrix, each row is a point in sphere.
% ind : If it is given, calculate the minimal distance between
% the ind-th sample and the other samples in grad.
% radius : covering radius, i.e., the minimal spherical distance between any two samples.
% 1. http://mathworld.wolfram.com/SphericalCode.html
% 2. "Designing Single- and Multiple-Shell Sampling Schemes for Diffusion MRI Using Spherical Code",
% Jian Cheng, Dinggang Shen, Pew-Thian Yap, MICCAI 2014.
% 3. "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 <email@example.com>