Support pad readout and APV setups
Various changes to support detectors with pad readout and reconstruct SRS APV data, as needed for the resistive-MPGD HCAL common project. I don't really want to go in detail on all the changes but git suggests these ones:
- Create classes DigiStrip and DigiPad and make Digi a base class
- Create classes ClusterStrip and ClusterPad and make Cluster a base class
- Support creating clusters and rechits from pads in DigiToRechit
- and I guess many more.