[VECGEOM-572] vgdml::Frontend::Load returns now the Middleware pointer...
[VECGEOM-572] vgdml::Frontend::Load returns now the Middleware pointer exposing material and region info.
The commit closes VECGEOM-572, exposing internal maps stored in the Middleware class after calling Frontend::Load with a GDML file. An example ReadMiddlewareData was provided to demonstrate in detail the usage of these maps.
Closes VECGEOM-572