Add mask parameter to unsharp

[steventgk]

Add a mask parameter to the unsharp code to allow for the removal of 'spots' in the final unsharp image.

E.g. if the input array is number counts, cond = stat>100 would be a mask of the same size, which has values of true where the counts are larger than 100.

We can then set !cond=np.nan to mask out statistically meaningless values in the unsharp image.