Image rescaling in foci analysis

[gletreut]

In the foci_lap method of mm3_helpers.py file, it might be worth it to scale the img_foci array before applying the foci detection. For a grayscale image, just apply the operation:
img_new = (img - px_min)/(px_max - px_min) * 255

This should help to treat all images equally for foci detection. In particular, instead of having the same threshold for all images, one could take:
threshold = px_median * snr,
where snr is the signal-to-noise ratio used, i.e. foci_log_peak_med_ratio.