Fixed bug in Model2DKernel
Added type cast to float64 in CustomKernel and adapted peak normalization
Added simple ModelKernel tests
Clarified MexicanHatKernel normalization
Raising exception when normalizing kernels, that sum to zero.
Merge pull request #2007 from adonath/fixing_issue_#1986

Fixing #1986 and a few other minor issues