Clean up mace3D and 3D shepp Logan demo scripts

Created by: dyang37

In mace3D and mace4D demo scripts:

• Remove prior_weight=0.5 in the demo, and specify in the comments that sharpness is the parameter we recommend to change regularization level of reconstruction.
• Remove dncnn_keras denoiser.
• Explicitly perform qGGMRF reconstruction in the demo, and remove the functionality of saving init_image to save_path in mace3D() function.

In 3D shepp Logan demo scripts:

• Change sharpness to 1.0.