Use caching of images in the Live Viewer for most recent images

[MikeSullivan7]

Desired Behaviour

Following from the work done with Dask, it may give a performance increase if we use a custom cache for the images read in by the Live Viewer for the most recent images. The cache should be fixed at a certain number of images which could be decided by the user depending on their memory resources. Using a cache would allow the spectrum to be calculated a lot faster for recent images, with the rest of the spectrum calculated for example by an async task.

Current Behaviour

The Live Viewer currently does not allow the spectrum to be calculated as the images are not stored in memory.