Add EffectiveSource code

[nilsvu]

Proposed changes

Bundles the EffectiveSource (https://github.com/barrywardell/EffectiveSource) code in external/ (GPL licensed). This is a widely used code in the self-force community to compute the effective source of a scalar point charge in Kerr spacetime.

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

[kidder]

why are we putting this into spectre and not just using it as an optional external dependency?

[nilsvu]

It won't ever change and is easier to bundle than to install as an extra dependency (in a recent effort to make the code easier to use we have been bundling other dependencies like libsharp as well). It's one .c file, and the original repo doesn't even have a build system, so it's easier to bundle the .c file.

[nilsdeppe]

Can you ask Barry if he'd be willing to relicense to something more permissive so we don't have to worry about GPL things again?

[nilsvu]

Ok I'll ask!