Merge multiscale and non-multiscale steps

This MR merges the multiscale and non-multiscale imaging steps (see RAP-281). It adds a new parset parameter called do_multiscale_clean and removes the old parameters sector_do_multiscale_list and multiscale_scales_pixel. Multiscale clean is now enabled by default, as it results in better deconvolution with only ~15-20% longer runtimes (for the full image workflow) in my testing. The way the WSClean threads are set has been improved, with a new parameter named parallel_gridding_threads added to the parset. By default, this parameter and deconvolution_threads are now both set to 2/5 of max_threads (which is passed to the -j argument of WSClean). Lastly, some general cleanup was done.