Draft: Find mat with XY avoiding ROOT caches

Closed as superseded by LHCb!4652 (merged) and !608

Adds a function to DeFTLayer to convert a position to a mat based on the geometry in order to run multi-threaded.

This is required as the existing functions rely on ROOT's TGeoNavigator cache which is not thread safe.

To do:

• Include stereo angle in calculation
• Veto the hole
• Find how to remove hard-coded geometry values
• Find reason for remaining gaps between mats and modules

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Validated by

• Core Software
• RTA
• Simulation