Tree ring test

@cwwalter requested in #127 a test to check the amplitude of tree rings in 1.2i. We have code from @karpov-sv here: https://github.com/karpov-sv/lsst-misc/blob/master/Tree_Rings_Analysis.ipynb

We'd need to adapt this code to use the e-image reader in GCR. @andrewkbradshaw, would you like to try to adapt this code to DESCQA?