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 e = error2d(discr, v1, v2)
    % If v1 and v2 are more complex types, something like grid functions... Then we may use .getVectorFrom here!
    h = discr.h;
    e = sqrt(h.^2*sum((v1-v2).^2));
end