Update a few task configs to support more stable processing of the imaging data. Details and justification for each change below.

• Change the default characterizeImage task configs to use a simple signal-to-noise starSelector for PSF candidate stars instead of the objectSize starSelector. The objectSize starSelector is not stable for auxTel images due to a lack of sources needed to form well defined clusters in the mag-size space.
• Switch to using default 'piff' for psf characterization. Previously we used the PCA algorithm for psf characterization as it was more stable with fewer PSF candidates, however the change to a signal-to-noise starSelector removes this limitation.
• Add assembleCoadd.py file for default configs when creating coadds. Since the auxtel PSF is not well defined, we use the WcsSelectImagesTask for selecting input images to the coadds, rather than WcsPsfSelectImagesTask. This means no filtering is done for poor/unacceptable PSFs before combination.