ITS: template Vertexer&Traits, IndexTableUtils

[f3sch]

resolves the last needed templetization for extending to more layers.
for now I commented building the anyways commented out files VertexerTraitsGPU and VertexingKernels out.
I have some code in a private branch which implements most of the vertexing stuff for GPU (although the last step is missing but Gabriele and I will figure out how to do it) so I will probably upstream this partial implementation at some point.

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[mconcas]

@davidrohr : changes to the GPU FW part are trivial, I'd merge.