Compute in subiters

[landmanbester]

Major changes:

• Replaced dirty and psf workers with a single grid worker to make dirty image, psf and compute weighted data.
• Added backward worker.
• Create datasets per gridding instance (split by field, ddid, scan and band).
• Everything now gets written to zarr files (fits files are optional and just for inspection).
• forward and backward workers call compute on hessian operators at each iteration to avoid unmanageable dask graphs.