[gdmlFile] optionally import physvol below top level

This allows syntax like
```xml
  <detectors>
    <detector id="1" name="FT_MT" type="DD4hep_GdmlDetector" vis="InvisibleWithChildren">
      <gdmlFile ref="GdmlDetector.gdml" physvol="lvUpstreamRegion_0"/>
      <parent name="/world"/>
    </detector>
  </detectors>
```
to only load the specific `physvol` in `examples/ClientTests/compact/GdmlDetector.gdml`.