Paramfix

Created by: dyang37

This branch includes some fixes to 3D shepp Logan demo, and some changes regarding default parameter values.

1. In cone3D.recon, changed default max_iterations from 20 to 100 to align with svmbir.
2. In cone3D.recon, changed default snr_db from 30 to 40. We need to do the same thing for svmbir.
3. In shepp Logan demo, changed value of T to 0.1 used snr_db=40 (current default value).

Tested and shepp Logan recon results look good.