Find dark angles

[lauraporta]

Is your feature request related to a problem? Please describe.
Create a method to identify where a given ROI gets "darker", i.e. the luminance is lost.
It is not trivial since this change could also be due to activity.

Describe the solution you'd like
This metric should be calculated via the incremental rotation dataset, because only there we are sure that every line is acquired at a given angle.
A way to estimate the luminance loss is by taking the mean at every 5deg position. It can be done taking in account total luminance or ΔF/F (although this would introduce a further confounder, see #11 ).