Type coherence

What should be the matrix type when having different types for the kernel and for the input data? For example :
```julia
kernelmatrix(k::Kernel{T1},X::Matrix{T2}) # where T1 is different from T2
kernelmatrix(k::Kernel{T1},X::Matrix{T2},Y::Matrix{T3} # where all Ts are different
```
Should we force the kernel one? Or should we let it free? In any case this is something we should watch!