Next-to-nearest neighbor in DEE charge transport simulations

[fhagemann]

Check if we need to simulate also next-to-nearest neighbors in the DEE charge-transport module.
They might not collect any charge, but they might be subject to spectator effects + cross-talk, and might be read out as "nearest neighbors" if the actual nearest neighbors surpass the energy threshold.

[fhagemann]

The way that #190 is handling this is to add "empty" strip hits (no energy, no timing -> would be slow timing anyhow) if a triggered strip did not have an associated nearest neighbor.

If we were to also implement next-to-nearest neighbor strips in MSubModuleChargeTransport, we would also have to parameterize the timing for these next-to-nearest neighbors (reminder: we're currently in the works of getting the nearest-neighbor timing correct, I guess no one is looking at next-to-nearest neighbors right now)