Develop metric for assessing an image's contribution to DcrModel

Current OpSim simulations suggest there are relatively few images per field in u & g in the first years of the LSST survey, which may make it challenging to construct usable DCR models.  This ticket is to develop an metric to characterize the relative value of a specific new image at a specific airmass and parallactic angle (given the past pointing history) in improving the DCR model and hence reducing the total number of DCR-induced false positives.