Make Stokes I-only predict mode work with array-factor beam

Created by: darafferty

Currently, predict cannot use the Stokes I-only mode (which predicts only Stokes I if the sky model is unpolarized) when the beam is applied, as the beam effects are generally non-scalar. However, if only the array factor of the beam is applied, then the beam effect is a scalar one. This pull request allows the Stokes I-only predict mode to work with the array factor beam, improving the speed by ~50% and decreasing the memory usage.