ImageDifferenceTask currently requires the output repo to be the same repo that contains coadd templates. This requirement is restrictive for AP, which isn't usually in the business of making templates, and would like to treat templates more like external calibration data and less like an intermediate pipeline product.
This ticket is to add a command-line argument for a third repo in ImageDifferenceTask so that the user may specify the location of science images, templates, and desired output location all independently.
(Note that when GetCalexpAsTemplateTask is invoked in lieu of using a coadd as a template, ImageDifferenceTask looks for the calexp template in the input repo. This is less restrictive for AP, but the ideal case here would also be compatible with the option to specify a third repo.)