Set default filenames for pixel XML
The pixel layout building relies on environment variables to determine which XML files it should load for building the various parts of the geometry. In some circumstances (such as when running with valgrind) these environment variables are not set when the tools are called and so the geometry building fails. The changes here set some default filenames to be used if the environment variables aren't available, such that at least the latest geometries can successfully be run with valgrind.
As an aside, this reliance on environment variables seems a very fragile system (as shown by the issue which prompted this change), and it would be better to set the filenames via jobProperties. However, since the classes responsible for reading the XML files don't derive from any Athena base class, this is not simple to do (would have to be passed through via a config object from a tool that can have properties set the normal way). Since we plan to deprecated this pixel framework in future, this is probably not the highest priority to do at the moment.