22.0-cleanup2-PixelDigitization

This is a re-structure of the EnergyDepositionTool in PixelDigitization, extracting the BichselData as a stand-alone class and introducing some unit tests for it (to be extended later). Some member functions were refactored to be file scope where possible. The existing code has been shuffled into the new classes with little attempt at optimisation (so far). During testing, some uncaught error conditions were found and code introduced to trap these.