Get neighboring pixel

This merge request is based on the issue raised here. Currently, Allpix Squared has a method to extract neighboring pixels that is hard coded within the source file and assumes a rectangular/square pixels. However, this is not compatible with other types of geometries that are currently being implemented, such as Hexagonal pixels and radial strips.

The new getNeighborPixels() is a new function in DetectorModel.cpp that obtains the neighboring pixels based on the specified induction matrix by the user in the TransientPropagationModule.cpp. This merge request will be about implementing the default geometry (rectangular/square), but it will be designed to be overridable for other sensor geometries that will be implemented in the future.

Implementation of getNeighborPixels() for Hexagonal sensor geometry will be worked on in a separate merge request.

This fixes #214 (closed)