HOWTO run hdgeant simulations using ccdb geometry

Until now, the hdgeant simulation depended on having the hdds geometry installed on a local filesystem. The geometry is represented by a set of linked xml files, with the top-level file (usually named main_HDDS.xml) referencing separate xml files for each subsystem as entity references. All of these xml files are now managed under a standalone github repository at https://github.com/JeffersonLab/hdds and installed locally at $HDDS_HOME within the standard GlueX environment. The files in the master branch of this repository are intended to represent the current operating state of the GlueX detectors and beamline. Other branches exist to provide a description of non-standard configurations, or exceptional setups that were present for special run periods.

Normally selection of the desired geometry to be used in a simulation takes place at the time the hdgeant binary is built. The standard build scripts first run "scons install" in HDDS_HOME, which translates the hdds geometry description into geant3-compatible functions, and builds them into the libhdds.so library. This library then gets linked in when the hdgeant executable is built during the "scons install" step under halld_sim. If you run hdgeant without special arguments (see below), that is the geometry that is used in the simulation.

There is a second method for feeding geometry information to hdgeant, which allows the simulation geometry to be selected at run time. This method is activated by the commannd-line option -xml. Here is an example of how this might be used.