Add surface tension

[wilfonba]

Adding surface tension feature a la Schmidmayer et al.

[wilfonba]

Steps to adding surface tension to MFC

• [x] Implement based on Schmidmayer 2017
• [x] Validate against 2D equilibration using WENO
• [x] Validate against 2D equilibration using MUSCL
• [x] Validate against 2D recovering a circle using WENO
• [x] Validate against 2D recovering a circle using MUSCL
• [x] Validate against 3D equilibration using WENO
• [ ] Validate against 3D equilibration using MUSCL
• [ ] Validate against 3D recovering a sphere using WENO
• [ ] Validate against 3D Recovering a sphere using MUSCL
• [ ] Validate against 2D shock-droplet using WENO
• [ ] Validate against 2D shock-droplet using MUSCL
• [x] Validate symmetry boundary conditions using equilibration and recovering a circle
• [ ] Determine run-time changes on GPUs