Account for origin in truth Lxy

Changes to code

Fix for a bug in the flavor labeling for b-jets: the truth particle decay vertex displacement wasn't accounting for the offset of the truth primary vertex.

This should fix it. There should be a small difference in variables that we save in PHYS.

Could probably use another look from @hartman and @svanstro.

Changes to output

I've attached the output from a diff in PHYS as diff-lxyfix.txt. The differences are (as expected) all in the Lxy variable:

 112|   o                                                       
 106|   o                                                       
 100|   o                                                       
  94|   o                                                       
  89|   o                                                       
  83|   o                                                       
  77|   oo                                                      
  71|   oo                                                      
  65|   oo                                                      
  60|  ooo                                                      
  54|  ooo                                                      
  48|  ooooo                                                    
  42|  ooooo   o                                             o  
  36|  ooooooooo         o          o                        oo 
  31|  ooooooooo         o   o o    o    o    o              oo 
  25|  ooooooooo  o   ooooo  ooo o  o  o o  o o oo   o       oo 
  19|  oooooooooo oo  ooooo  ooo oo o  o o  o o oo   o   ooo ooo
  13|  oooooooooo ooo ooooo  ooo ooooo o oo o o ooooooo  ooooooo
   7|  ooooooooooooooooooooooooooooooooooooooooooooooooo ooooooo
   2| oooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
     ----------------------------------------------------------
     - - - - - - - - - - - - - - - 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
     0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . . . . . . . . . . . . . . 
     . . . . . . . . . . . . . . . 0 0 1 1 2 2 3 3 4 4 5 5 6 6 
     7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 2 7 2 7 2 7 2 7 2 7 2 7 2 7 
     2 7 2 7 2 7 2 7 2 7 2 7 2 7 2 5 5 5 5 5 5 5 5 5 5 5 5 5 5 
     4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 8 8 8 8 8 8 8 8 8 8 8 8 8 8 

-------------------------------------
|              Summary              |
-------------------------------------
|         observations: 1591        |
|        min value: -0.724152       |
|          mean : -0.127803         |
|        max value: 0.717026        |
-------------------------------------

Overall this change reduces the value for Lxy, which is what we expect. It suggests that the truth vertex offset (for this sample) was probably around 0.65 mm.