Code structure of the Tractor

Miscellaneous notes on control flow and call stacks.

Tractor.optimize()
    Tractor.getDerivs()
        Image.getParamDerivatives()
        Tractor.getModelImage()
        Tractor._getSourceDerivatives()
            Source.getParamDerivatives()
    Tractor.getUpdateDirection()
        Tractor.getChiImage()
        scipy...lsqr()
    Tractor.tryUpdates()
        Tractor.getLogProb()

Tractor.getLogProb()
     Tractor.getLogPrior()
     Tractor.getLogLikelihood()
         Tractor.getChiImages()
             Tractor.getModelImages()
                 Tractor.getModelImage()
                     Image.getSky()
                     Tractor.getModelPatch()
                         Tractor.getModelPatchNoCache()
                             Source.getModelPatch()