Reduce a mask to nomask when possible.
Examples
>>> x = np.ma.array([[1,2 ], [3, 4]], mask=[0]*4) >>> x.mask array([[False, False], [False, False]], dtype=bool) >>> x.shrink_mask() >>> x.mask False
numpy.ma.MaskedArray.soften_mask
numpy.ma.MaskedArray.unshare_mask