GeoModelIO: New method to retrieve custom tables from the SQLite .db file

Currently, we have methods to write auxiliary tables into the SQLite file in standalone code, through the use of the GeoModelIO publisher.

And we have ways to retrieve those auxiliary data from within the ATLAS framework Athena, through the use of the IRDBAccessSvc interface.

However, we don't have code to retrieve those auxiliary data custom tables from a piece of standalone client code.

This MR introduces a new method into GeoModelIO::GeoModelRead, which lets users to retrieve any custom table from the SQLite file based on the table's name.

It also adds an example of how to do that in the GeoModelExamples/HelloToyXML example.

This MR closes #57 (closed)