GEC-94 Do not touch main buffer in onlypredict+keepmodel mode

Leave the main data buffer intact when using onlypredict=true and keepmodel=true. Following DDECal or BdaDdeCal steps can then use both the predicted data in the model data buffers and the measured visiblities in the main data buffer.

Since BDA predictions are the fastest when first doing expansion of the BDA data, than the actual predictions, and then re-averaging of that data, predictions inside BdaDdeCal always include BdaExpander and BdaAverager steps. This MR allows doing the predictions in DDECal, without extra BdaExpander/BdaAverager steps. A single BdaAverager step can then average the data and feed it to BdaDdeCal, which doesn't need to do any predictions anymore.

Since there are probably no users that use onlypredict=true and keepmodel=true, changing the behavior shouldn't hurt for this combination. If you think it does, and we need a transition / deprecation path, please let me know.