Mace3D improvement

Created by: dyang37

This PR is mostly handling issue #65 (closed) and #66 (closed). This PR contains improvements to mace3D demo scripts as well as mace functionality:

1. Remove keras denoiser from mace3D demo scripts.
2. In mace3D demo scripts, explicitly perform a qGGMRF recon. Previously this was done by loading the numpy array that contains qGGMRF recon, where the array is saved by mace3D function when computing qGGMRF as an init image.
3. Remove os operations in mace functions: In mace3D and 4D we no longer write any files to disk.
4. In mace3D and mace4D, change qGGMRF sharpness to be the same as MACE sharpness.
5. Some optimization to mace4D dask variable initialization. We only initialize dask variables when a cluster_ticket is provided.
6. Some minor updates on data download function.