Add BnsInitialData System

[isaaclegred]

Proposed changes

This PR adds the system for generating BNS Irrotational Initial data. To do this, an elliptic system is introduced to solve for the velocity potential using the framework of Baumgarte + Shapiro Ch. 15. The background consists of a matter profile which is not yet implemented (as it is contained in the initial data).

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

[isaaclegred]

@nilsvu whenever you get a chance!

[isaaclegred]

I think everything should be done except removing the "flat" things. I'll do that in a separate commit after squashing all of this because I plan on keeping it around on a branch just in case it becomes useful for checking that the executable is working .

[isaaclegred]

@nilsvu I renamed everything to BnsInitialData in order to make it more general. Also fixed up comments.