AST-1266 Fix ddecal bug with model data
Both DDECal and GainCal pass their original buffers to the step that read the model data column, resulting in the data being overwritten. Since the data and model data are then the same, the solver converges in one iteration.
This MR extends the integration test of DDECal and GainCal to check for this condition.
Both DDECal and GainCal are fixed by passing a copy of the buffer to the step reading the model data.
Edited by Bas van der Tol