Merge shape + photometry masks by intersection; build N(x)/randoms mask #837

Combine the shape and PhotoPipe masks by intersection. Build the N_exp>=2 and N_point>=3 masks (N_point already in ShapePipe). The CCD-failure-based N(x) / randoms mask (where galaxies are not, for clustering/void work) is the missing piece.

Context: Paris meeting notes