HEALPix pixel plotting improvements

[smcguire-cmu]

When calling plot_pixels it would be nice to have the following features:

• Plot additional values on top of healpix map

The ability to add other plots such as shapes or points on top of the healpix plot. It would be ideal to have access to the Matplotlib axes, or to plot on a specified axes instead of making and rendering a new plot.

• Show borders between pixels of same order

With multiple pixels of the same order, it is hard to see the boundary of the individual pixels. Displaying the pixel borders as a line as well as the coloring in based on order would be helpful.

• Axis labels

Having the axes labeled with tick marks showing values would be useful to see.

• Discrete Legend for one order

For pixel order plots where a single order is plotted, the legend shows a continuous color change from order - 1 to order + 1. For more than one order, a discrete legend is displayed with a block of color for each order in the catalog. The single order legend being discrete also would be better.