Add preprocessing option to select the dF/F method

[crnolan]

Currently the default dF/F calculation is to detrend with a high-pass filter and divide this by the low-pass filtered signal (after removing artifact). This works to remove bleaching, but also removes any (legitimate) slow changes in the signal. We need to improve this, but also perhaps just allow the method to be selected via a configuration option (and in the preprocessing GUI for data exploration).