Reapply "Peak value is relative to mean (thanks Mads)"

This reverts commit 586b0d2f.

src/calng/kernels/peakfinder9_gpu.cu

@@ -149,7 +149,7 @@ extern "C" __global__ void pf9(const unsigned short num_frames,
 	// whole peak should have sufficent SNR
 	float peak_weighted_row;
 	float peak_weighted_col;
-	float peak_total_mass = pixel_val;
+	float peak_total_mass = pixel_val - mean;
 	{
 		/* TODO: more compact form */
 		float peak_weighted_row_nom = static_cast<float>(row) * pixel_val;
@@ -159,9 +159,10 @@ extern "C" __global__ void pf9(const unsigned short num_frames,
 			float total_mass_before = peak_total_mass;
 			masked_frame.fun_ring(row, col, layer, [&] (unsigned short i, unsigned short j, float val) {
 				if (val > peak_pixel_threshold) {
-					peak_total_mass += val;
-					peak_weighted_row_nom += val * static_cast<float>(i);
-					peak_weighted_col_nom += val * static_cast<float>(j);
+					float val_over_mean = val - mean;
+					peak_total_mass += val_over_mean;
+					peak_weighted_row_nom += val_over_mean * static_cast<float>(i);
+					peak_weighted_col_nom += val_over_mean * static_cast<float>(j);
 				}
 			});
 			// in case nothing was added, stop expanding