I have a warning which should prevent users to normalize a kernel, when the kernels sum is close to zero.
As this is the case for `MexicanHatKernels` the warning is catched here, because I considered it strange that 
a built-in kernel raises a warning. But this is becasue I have the warning in the base kernel class constructor ...It may be better to have it somewhere else e.g. in the `normalize()` function.