function potential: dispatch on type

[sbadrian]

I was wondering if there is a reason why the type can be provided as a keyword argument. I think in principle is suffices to use a promote_type on scalartype(op) and eltype(coeffs).

If there is nothing withstanding, I can change the behavior.

[krcools]

scalartype does not always give the correct result. For example the Maxwell single layer potential takes on SVector{ComplexF64}, whereas its scalartype is simply ComplexF64. There should be a valuetype similar to scalartype and implemented for all potentials to fix this. Because that's not there (yet) I went for this amittedly ugly fix.