WindowedGaussianBackgroundMapper computes incorrect outer window mean

[tboggs]

Inner window mean is incorrectly computed, which causes outer window mean to be incorrect when cov argument is specified. This error affects results from rx, ace, and matched_filter, when the cov parameter is specified. Qualitatively, the resulting errors of downstream algorithms appear small and results on relative scores of pixels within images are mostly imperceptible, particularly where pixels are more anomalous.