Add BeamSpot retriever to InDetRetrieversCfg (CA config)

Add BeamSpotRetriever data-type into getDataTypes, add BeamSpotRetrieverCfg configuration function and merge its output Component Accumulator into the Inner Detector retrievers configuration (InDetRetrieversCfg config function). This adds the following BeamSpot position information of the PV to the output xml, for example:

> <BeamSpot count="1" storeGateKey="BeamSpot">
> 
> <label>
> Beamspot_position_at_PV_z_position 
> </label>
> <sigmaX>
> 0.00796396 
> </sigmaX>
> <sigmaXY>
> 3.45524e-06 
> </sigmaXY>
> <sigmaY>
> 0.00857973 
> </sigmaY>
> <sigmaZ>
> 36.4616 
> </sigmaZ>
> <status>
> 59 
> </status>
> <tiltX>
> -4.18488e-05 
> </tiltX>
> <tiltY>
> -5.09917e-05 
> </tiltY>
> <x>
> -0.64885 
> </x>
> <y>
> -0.233964 
> </y>
> <z>
> 3.72128 
> </z>
> </BeamSpot>

Tagging @jferrand @myexley @lshi @emoyse for info/comment.