Add copy_prior_of_unattached function for intercal

Use this function to copy prior zeropoint offsets to fields that are
unattached to the network, and thus do not get intercal zeropoint
solutions. Useful for sky fields, for example.
analyze_network removes edges that have no overallping stars

A NoOverlappingStars exception is thrown when an insufficient number of
overlapping stars is seen in an edge. This edge is then deleted.