VPCluster dumper tool for GPU studies

FYI : @dovombru , @dtou , @raaij , @gligorov I added the PrPixelMCTools package used to dump the Hits to the N-tuple used for the Dorothea studies. I put this in merge request to the vp_gpu branch.

By construction the tuple is filled storing arrays of Hit position information (also LHCbID and cluster information on sensors,module etc..) for each reconstructible Velo track. The same information is stored on a "Dummy" track for hits not associated to reconstructible velo tracks. Given that, you can re-shuffle the tuple produced offline to have a per-event hit container to be used for pattern recognition studies and fully develop offline a tracking algorithm (search windows study etc...).