[Halld-offline] update to HDGeant4: geometry from ccdb

[Richard Jones]

Dear Colleagues,

As announced at this week's collaboration biweekly mtg, HDGeant4 has been
upgraded to be able to get its geometry either from xml files or from ccdb.
Just like in hd_ana, the source of the geometry is determined by the
combination JANA_GEOMETRY_URL and JANA_CALIB_CONTEXT, as follows.

   1. If JANA_GEOMETRY_URL starts with xmlfile:// then geometry is read
   from plain text xml files on a local filesystem. Until now, this was the
   only option supported by hdgeant4.
   2. If JANA_GEOMETRY_URL starts with ccdb:// then the geometry is read
   from ccdb, using the run number being simulated and the variation specified
   in JANA_CALIB_CONTEXT (or mc_generic if JANA_CALIB_CONTEXT is not set).
   This is new capability, as of now.
   3. If JANA_GEOMETRY_URL is something else or not set, hdgeant4 exits
   with an error. This is the same behavior as it had before.

WARNING - this update required a minor patch to JANA, which David has
released as jana version 0.7.9p1. If you have not installed this patch, do
not update your local build of HDGeant4 as it will not compile.

-Richard Jones
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