Return nan as Stage if Input is NaN

[aegis301]

Right now, if we use a threshold for the imputation of values that are missing the interpolater correctly omits interpolating values and returns nan. However, the Probe, when encountering a full set of nan values still returns AKI stage 0. However it should return np.nan instead.

[aegis301]

If then all stages are np.nan, we should return np.nan in the total stage, too