fix mri_convert produced gradients

fix_bvecs.py

img = nib.load(imgfile)
bvecs = np.genfromtxt(bvecs)
    
def get_trackvis_mat(aff):
    """based on input from ruopeng @ tracvkis
    """
    for i in range(10):
        #aff = aff.dot(np.linalg.inv(np.eye(3) + 3*aff.T.dot(aff)).dot(3*np.eye(3) + aff.T.dot(aff)))
        aff = 0.5 * (aff + np.linalg.inv(aff.T))
    return np.dot(aff, np.array([[1,0,0],[0,-1,0],[0,0,1]]))

if rotate:
    mat = get_trackvis_mat(img.get_affine().copy()[:3, :3])
    bvecs = mat.dot(bvecs.T).T