Converging source

[jacobfeder]

Add a "ConvergentSource" such as what might be coming from an objective lens.

[JLBegin]

Thanks for your contribution! I’ve taken your original commit and made a few refinements, including:

• Replacing the focal_point vector with a focalLength, paired with the parent direction property, for a simpler and more consistent, symmetric photon convergence. focalPoint allowed photons to point off-axis, whereas typical use cases will assume convergence along the source axis. For off-axis behavior, a custom source class can still be implemented by the user.
• Added unit tests.

I’ve opened #142 which includes your commit plus these updates, which may be merged into master.