Sphere2 -> Unit3

[Frank Dellaert]

@atrevor @jdong37 @richardroberts I am thinking about renaming Sphere2 to Unit3. Sphere2 is the space, rather than the object, and the current name is a bit confusing and also inconsistent with the current naming conventions like Rot3, Pose3. Since we have not yet released anything on this, I propose to do so in 2.4, then @richardroberts can merge into 3.0

Respond fast if you think this is a particularly bad idea

[Alex Trevor]

I agree that Unit3 is a better name for this than Sphere2.

[Frank Dellaert]

OK, I did this. @richardroberts we can now merge into develop. It is a bit tricky, I think, so maybe we can Skype and do this together? Let me know by email when you could do it.

[Chris Beall]

Also renamed test in commit f0257ee