Test Suite Fails to Catch Errors in .png Images

[geogunow]

Currently we use an RGB histogram to determine whether plots are equivalent. This was done to get around the issue of .png binary files not being exactly the same due to dependence on the local installation but being the same in all practical purposes. However, the RGB histogram method currently appears to not report significant differences in .png files as noted in #296. We should work to improve the test suite so that these differences are caught and therefore errors are correctly reported.