[Halld-offline] new itrack info saved in MC hddm truth tags

[Richard Jones]

Hello all,

As requested by Justin, I have just checked in an update to the simulation
and hddm decoding libraries in sim-recon to introduce itrack information
for TruthShower and TruthPoint tags. Formerly only a "track" index was
provided.

   - "track" - a unique ID assigned by geant to each particle it tracks,
   including primaries and secondaries generated outside the calorimeters or
   cave.
   - "itrack" - a lookup index that can be used to find the particle in the
   MC record that gave rise to the tracking point or shower, or -1 if it is a
   secondary.

With this change, you get both track and itrack for each TruthPoint and
TruthShower. The new code is backward-compatible, meaning that hddm files
that were generated prior to this update will still be readable by the new
code (albeit with itrack=0 being read everywhere it was missing before) and
that hddm files generated using the new code are still readable by old
consumer binaries (albeit without access to the new itrack info).

I am now propagating all of these changes into the commissioning branch -
what a pain!!

-Richard J.
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