Added function to calculate activity overview image.
Added function to calculate activity overview image.

The activity overview image shows, for each pixel, the variance of the
correlation of this pixel with its neighbors. Assuming that this value
is lower for the neuropil than for active cells, this gives an overview
of where active cells are. Works with the pixCov matrix returned by
calcPxCovMJLM.

HarveyLab/Acquisition2P_class,HarveyLab/Acquisition2P_class