mace 3D & 4D PR

Created by: dyang37

This is a pull request to merge mace3D and mace4D into master.

To test:

1. checkout pr/mace4D branch: git checkout pr/mace4D, git pull
2. Install requirements.txt: pip install -r requirements.txt
3. Re-install mbircone: pip install .
4. Install demo related packages: cd demo, pip install -r requirements_demo.txt
5. Clear system matrix cache: rm -r ~/.cache/mbircone/
6. Run demo:

• mace3D: python demo_mace3D_fast.py
• mace4D: python demo_mace4D_fast.py

Both demo should take about 5-10 minutes on cluster, and 15-20 minutes on a personal computer.

In addition, if you have access to the cluster and would like to perform a higher quality demo, you could run demo_mace3D.py and demo_mace4D.py (basically 3D/4D reconstructions with higher resolution phantoms).