Add transformation to tortoise radius

[nilsvu]

Proposed changes

Bundles some root-finding code from https://github.com/tosburn3/KerrGeodesicsC to transform between Boyer-Lindquist and tortoise radius. The bundled code has figured out good initial guesses and a robust method for the root find.

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

[wthrowe]

I think this is a license violation. The GPL would let us distribute a verbatim copy of the upstream package, but I don't think we can distribute a version that has been modified like this as part of SpECTRE.

[nilsvu]

My understanding is that this is a redistribution of the GPL-licensed code with some changes still under the GPL license, which is ok. The changes are: Deleted some functions to avoid dependence on FFTW, added a CMakeLists.txt to compile the code with CMake, added a C++ compatibility header, and auto-formatted the code. These are listed in the README, and the license is also included.