Fix total_flux computation

[AlexKurek]

Background noise within the bounding box (but outside the island) was included in the island's total flux. To fix this, we mask the area outside the island.
There are a lot of differences in the .gaul file after this.

[AlexKurek]

This is how self.image[in_bbox_and_unmasked] currently looks. It confirms, that total flux of the bounding box is computed, not the total flux of a source.

Adding this:

Its primary use is a container for all kinds of data describing island.
    """
    _img_save_count = 0

    def __init__(self, img, mask, mean, rms, labels, bbox, idx,
                 beamarea, origin=None, noise_mask=None, copy=False):

and this:

        self.islmean = bbox_mean_im[in_bbox_and_unmasked].mean()

        if Island._img_save_count < 5:
            import matplotlib.pyplot as plt
            plot_data = N.full(self.image.shape, N.nan)
            plot_data[in_bbox_and_unmasked] = self.image[in_bbox_and_unmasked]
            
            plt.figure(figsize=(6, 5))
            plt.imshow(plot_data, origin='lower', cmap='viridis')
            plt.colorbar(label='Flux')
            plt.title(f'Island ID: {idx}')

            plt.savefig(f'island_{Island._img_save_count}.png', bbox_inches='tight')
            plt.close()
            
            Island._img_save_count += 1

        self.total_flux = N.nansum(self.image[in_bbox_and_unmasked])/beamarea

will show:

[AlexKurek]

After this PR:

Tagging @gmloose since you may be interested to see this.