Draft: Load subdetectors

Create "LHCb.xml" file for some specific subdetectors. Thus we don't need to load the whole detector for subdetector studies.

I move this function in Gauss (https://gitlab.cern.ch/lhcb/Gauss/-/blob/master/Sim/Gauss/python/Gauss/Geometry.py?ref_type=heads#L455-579) to Detector, so that we can call it in other projects, eg. Boole. We hope to maintain this function only in Detector project.

This draft is preliminary. Following issues remain to solve:

• Paths to some subdetectors are defined in different ways
• Find a good way to call this function in other projects (eg. Boole!752)
• Compatibility for Gauss

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Validated by

• Core Software
• RTA
• Simulation