This value is more or less arbitrary (it should just be small enough). I wanted that the kernels have a truncation attribute, which decribes how much of the flux (if you have an astronomic image) is "lost" due to the limited size of the mask. This is only important for kernels that have an infinite response function. I had also in mind that this could be important for a future PSF class (e.g if I you have an analytical PSF model). 

The problem is that a Mexican Hat kernel sums up to zero. And in this case the definition of the truncation
(deviation of the masks sum from 1) doesn't make sense. Maybe it would be more reasonbale to define that for the MexicanHat seperately in the corresponding class, instead of making a distinction here.