Made correctLineShift more robust (but less accurate).
Made correctLineShift more robust (but less accurate).

CorrectLineShift used to estimate the line shift for each frame. This
didn't work well for dim movies, where each frame does not contain
enough information to estimate the line shift robustly. Now, the line
shift is estimated on the temporal average of the entire movie. Since
the line shift does only changes slowly, and movies are at most a few
minutes long, this seems a better solution.

HarveyLab/Acquisition2P_class,HarveyLab/Acquisition2P_class