TRTDetectorFactory_Full: changed the way the central barrel is constructed

The goal of this exercise is to test a different approach to the construction of the central barrel. If this mechanism proves to be successful, then a similar strategy will be applied to endcaps, and finally, it will also be implemented in the standalone TRT GeoModel plugin code in the GeoModelATLAS repository.

In the current implementation, we construct one instance of the barrel radiator Serial Transformer per layer for each potential straw material - Xenon, Argon, and Krypton - and then reuse it as many times as needed when constructing the barrel. In the new approach, we don't share Serial Transformers. Instead for each radiator, a new Transformer is created. The memory overhead of the new implementation is negligible (at the level of 0.5% wrt the current one). OTOH, after applying this strategy to the standalone implementation it will be possible to patch the materials of individual modules after reading the persistent description from SQLite.

CC @cgrefe