Allow multiple ITk Strip DetectorElements per simulated SensitiveDetector

Adds functionality to allow a single simulated ITk strip sensor to be split into multiple DetectorElements in the ReadoutGeometry. This is useful for ITk where we want each row of a Strip sensor to be a DetectorElement, with its own tracking surface bounds, etc.

The splitting is triggered by the presence of splitLevel parameters in the Xml geometry description. If these are not present, each simulated SensitiveDetector will become a single DetectorElement (with multiple rows potentially), as in the current scheme.

This has been tested in 21.9 (see !40125 (merged)) and found to work as expected.