Improve initial image parameters

This MR tweaks a number of WSClean parameters used in making the initial image to generate a starting sky model: the number of output channel images (-channels-out) is now limited to a maximum of 8 (and a min of 4) and the maximum number of major iterations (-nmiter) is set to 8. The new values should improve the speed of the imaging step, especially when processing large bandwidths. See RAP-855 for further ideas to improve the speed of this step.