Non-finite z-score scale is refused as 'spread is zero' #70

normalization._parameters refuses not np.isfinite(scale) or scale <= 0 with one message: "its intensity spread is zero (all pixels have the same value)". A non-finite scale (for example the std of a float64 plane of ±1.7e308 overflowing to inf or nan) is not a zero spread, so the message misdirects the user. Split the two cases and name the overflow (found while reviewing #69; it is the same on main before #69).

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