Overlapping ECORR parameters for EcorrKernelNoise #422
Description
The PPTA models ECORR in such a way that multiple ECORR parameters can apply to the same TOA. That means that the noise matrix is no longer block-diagonal with the blocks being diagonal + a rank-1 update. It's now a rank-k update, so ShermanMorrison does not apply.
Such a noise model will require a more general ShermanMorrisonWoodbury update. ShermanMorrison can be adjusted for this. For computational efficiency, we might need to check whether the logic to decide which to use needs to be done inside the for-loops, or whether we need a simple class for ShermanMorrison and a more general class for the rank-k ShermanMorrisonWoodbury version.
Currently we do not have checks in place to see whether ShermanMorrison is capable of the model, so I am listing this as a bug.