[v2.3-stable] DetectorField: Do Not Extend Into Sensor Excess

While this has no impact on the simulation since charge carriers outside the pixel matrix are ignored, it still is confusing to find that the electric field is replicated into the sensor excess defined in the detector model (see https://allpix-squared-forum.web.cern.ch/t/custom-tcad-electric-field-for-alpide-chip/310/7 for reference).

This part of the code has been reworked completely in master, but to fix this in v2.3-stable here is a patch adding the missing checks for the pixel matrix boundary.