Refine centroiding/spot detection algorithms based on commissioning run data

Based on the data from the commissioning run the centroiding and spot detection algorithms need to be updated for robustness and accuracy with strongly non-Gaussian PSFs and variation of the PSF shape over the field.

Using SeXtractor + PSFEx with PSF fitting provides more than sufficient accuracy and robustness, but is much too slow for on the fly analysis. Therefore, I am adapting SeXtractor methods to find a balance between accuracy/robustness and speed.