Baseline images for testing

[wtgee]

In panoptes/POCS#735 I have added pytest-mpl and a basic test of the dithering plot. This requires that we have baseline images against which to compare when the tests are run. For now I have just placed this directory alongside the test (in pocs/tests/utils/baseline_images) but this could feasibly get out of control if we really started testing a lot of images.

You can use a url for the baseline image or you can do some tricks on the command line to point to various other places, e.g. a repo dedicated just to holding images and artifacts.

There is no real problem right now with how this is done but I wanted to capture the potential issue.

Related to #112