Avoid fragmentation of calibration parameters
Calibration is a bit of an unusual operation in that it is not memory bandwidth, but actually computationally bound. For that reason, and because the operation uses a comparatively large amount of data in the time direction to calculate a relatively small number of parameters per grid cell, we chunk the input for the calibration option in fairly small tiles in grid space, suitable for speedy calculation and high parallelization. However, once the calibration has been carried out, these tiles are two small for efficient further calculations, so we should add another rechunking step.