This adds imageDifferenceTask and ForcedPhotCcdTask configured to use image differences to the DRP pipeline. 

It requires multiVisit to have been run (for deepCoadd_ref) and requires makeWarp to have been run. 

It remains a separate subset "diffimDRP" so that we can optionally run it as part of the DC2/RC2 reprocessing separate from multiVisit. 

Confirmed that it runs on: ci_hsc_gen3, RC2, and DC2. 

I needed to make edits to the GetCoaddAsTemplateTask. It was bypassing Gen3's capability for spatial queries and was still querying the skymap which was giving it wrong answers. For some CCDs in tract=9813, more of the ccd overlapped 9812 per the skymap, and it was filtering out all the coadds. The gen3 implementation of GetCoaddAsTemplateTask needs an overhaul but thats for a later ticket. 

In order to get ci_hsc_gen3 to pass I excluded the 2 detectors which we expect to fail difference imaging and forcedPhotDiffim from the entire quantum graph. 

jenkins running here https://ci.lsst.codes/job/stack-os-matrix/34012/display/redirect and I'll manually run an ap_verify 

See minor comments on PR.