Ignore nans when calculating mean images.

The meanRef and lucasKanade function used to propagate nans in one frame
to all frames when averaging. If a single frame had a large motion
correction shift and thus many nans, all those pixels would be blanked
in all averages. This commit fixes this.
Added Matthias' version of pixel covariance calculation.

This function calculates the covariance within a square neighborhood for
all pixels. The output can be used by selectROIs.
Added aux funs for Matthias.

These functions might be deleted or moved into the common code base
soon.
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.