Add intersectionWithSphere function

[SuzzzWood]

Added intersectionWithSphere function Maths from wikipedia

[ianmackenzie]

Looks good! (And don't worry about the Travis build failure - I checked and it's an unrelated test that's failing, one that fails occasionally and I have to look into at some point but not related to any of this work.)

I was going to make a couple minor tweaks but I'm not sure I have write access to your axis-sphere-intersection branch - is the "Allow edits from maintainers" box checked? The changes are basically:

• Destructure values using (e.g.) (Types.Vector3d cto) = ... instead of (Vector3d cto) = ... - this is the pattern I've used elsewhere, primarily because it seems weird to both expose the Axis3d type from the Types module and have an Axis3d type alias in the Axis3d module itself
• Use a couple functions to simplify the code: Vector3d.componentIn instead of dot to avoid the awkward direction/vector conversion, and Point3d.along to simplify the final point construction

[SuzzzWood]

@ianmackenzie thanks for the feedback! Yes, 'Allow edits from maintainers' is checked, feel free to edit

[ianmackenzie]

Just added a fuzz test for axis/sphere intersection - everything passes, so this looks good!

One last change: could you add yourself to the AUTHORS file? =)

[SuzzzWood]

Nice! Have added, thanks