ENH: Never return negative uncertainty; return NaN instead.

This is a work in progress.

The Savin-Doyle procedure for estimating uncertainty fails in dense systems or images with distinct background features. This often leads to 'signal' being negative. Instead of returning a negative value for the uncertainty 'ep', we should return NaN to make it clear that an uncertainty could not be estimated.

The tests for this need to be designed carefully, and this PR is not there yet.