Introduce source class controls for catalogs

[cdeil]

There's a lot of very bright sources on this low-energy Fermi-LAT images that we're currently using, that don't show up in the 2FHL catalog. Some of them are even blue, i.e. bright in the highest energy band on the image used.

I clicked on some of them, and as expected they are all pulsars (which have a steep cutoff in the spectrum).

It would be very nice to be able to select sources by souce type like at
http://fermi.gsfc.nasa.gov/ssc/data/access/lat/4yr_catalog/3FGL-table/#aitoff
or color the markers by source type.

This is just a reminder issue for this feature, I don't have specific suggestions yet for what exactly I would like to have or how it should be implemented.

[cdeil]

This is another interesting example. There's a cluster of 3FGL sources below the Galactic plane. What are they? Are they all unknown and low significance, i.e. maybe even fake? Or are there high significance and associated sources?

It would be great if one could investigate such questions by being able to display more information about the sources somehow (e.g. change marker colors or symbol or something ... it's not easy to implement well.)

One improvement could be that one (optionally?) displays source info on hover instead of on click, to let the user more quickly browse sources and see the popups.