Retuning in tune_local_threshold.py

The normal (non-meta) tuning script so far cannot handle retuning. It just scans over all possible TDACs and finds the optimum for each pixel. For the linear FE, this works well down to ~1500e threshold. At the moment, there is no good solution for tuning to thresholds between this 1500e and the noise floor using the noise tuning. This should by done by retuning, which should be handled by either an independent script or by tune_local_threshold.py