Extend ROI plotter

[ericdill]

Extend the ROI plotter (https://github.com/Nikea/xray-vision/blob/master/xray_vision/mpl_plotting/roi.py#L5) so that it can plot the image with the ROIs overlayed on top

[edit] can't spell

[ericdill]

In theory, one could do something like

fig, ax = plt.subplots()
ax.imshow(masked_sum_img, cmap='CMRmap_r')
mpl_plot.show_label_array(ax, rings_mask, cmap='gray', alpha=.1)

but, in practice that doesn't really work that well. To do this correctly, some more thought is needed

[tacaswell]

I would suggest using the label array as a binary mask and plotting the image with the non-labeled pixels set to np.nan, plotting the image in grayscale and the over-plotting the regions with partial alpha.