In Curve: Added numerical FD differentiation if derivative is not supplied. Added arc length computation based on the derivative. Added arc length parameterization (but this function is very slow.). In +util: Added fzero_vec.m and integral_vec.m, which call fzero and integral but take vector arguments.

function default_field(s, f, val)
    if isfield(s,f)
        return
    end
    s.(f) = val;
    assignin('caller', inputname(1),s);
end