re-enable vertex reconstruction on the GPU

[fwyzard]

• revert Disable vertex reconstruction on GPU #177
• "keep" pixelVertices in the profiling workflows:

diff --git a/RecoPixelVertexing/Configuration/python/customizePixelTracksForProfiling.py b/RecoPixelVertexing/Configuration/python/customizePixelTracksForProfiling.py
index 99a3a93..e1f0f9c 100644
--- a/RecoPixelVertexing/Configuration/python/customizePixelTracksForProfiling.py
+++ b/RecoPixelVertexing/Configuration/python/customizePixelTracksForProfiling.py
@@ -6,6 +6,7 @@ def customizePixelTracksForProfiling(process):
     process.out = cms.OutputModule("AsciiOutputModule",
         outputCommands = cms.untracked.vstring(
             "keep *_pixelTracks_*_*",
+            "keep *_pixelVertices_*_*",
         ),
         verbosity = cms.untracked.uint32(0),
     )

• disable if the transfers are disabled, because it relies on pixel tracks from the CPU

[fwyzard]

done by #197 and #216