Implement the visual display of MOC data in Firefly.

1. Initially, display the HEALPixels actually identified in the MOC (which is just a list of HEALPixel IDs in NUNIQ format) as polygons overlaid on a sky display in Firefly. Include this in the usual Firefly layer control behavior.
   • In this as in other cases, it would be useful to be able to visualize a MOC in the absence of associated image data - i.e., to have the image layer turned off or missing.
   • We would like multiple MOCs to be able to be displayed at once, with selectable colors used.
   • It may be useful to enable the control of both polygon boundary colors, transparency, etc. as well as polygon fill with adjustable transparency.
2. Later, it would also be useful to:
   1. Make the MOC pixels selectable so that they could be used in query operations to retrieve related data.
      • This is closely related to, but not identical with, the ability to make pixels in a regular HEALPix grid layer selectable.
   2. Enable set-logical operations on combinations of MOCs, allowing the display of set operations such as intersections and unions of MOCs.
   3. Enable the display of MOC data overlaid on x-y plots where the axes are in sky coordinates.

(This is a separate task from the ability to use MOC data in the back end of a data search engine to allow the search engine to rapidly tell a user whether there is catalog or image coverage available from a given data source / mission / survey for a particular location or region on the sky.)

Note- some but not all of the above has been broken into child task.