If a single session is used for a long time, HEXRDGUI starts using a lot of RAM

[psavery]

Rachel Lim reported that after she used HEXRDGUI for several hours, it filled up all of the RAM on her mac.

I know matplotlib sometimes keeps global references to objects. Perhaps we can make sure that when we discard/remove old canvases and artists, that they actually get cleaned up in memory.

[rachelelim]

I've been suspecting that it might be an issue with matplotlib now clearing memory when zooming in and out of images a bunch. The other possible culprit is something in pull spots as far as I have been able to tell