Add MeshBool::sphere()

[JaminKoke]

So I wanted to add sphere for the test suite, but it was bigger than I though, and it is also tripping an assert when I run it with the test suite.

[ RUN      ] Properties.ToleranceSphere

thread '<unnamed>' panicked at src/meshboolimpl.rs:176:13:
topological degeneracy
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

As of now I am unable to figure out why it's not working, so if you happen to notice something I missed, let me know!
If not, no worries. I'll keep working on figuring it out.

I think maybe this is possibly ready now! (Read latest comment) I have not really tested it very well.

[JaminKoke]

Whoops

[JaminKoke]

[JaminKoke]

I believe this is all good now! I just removed rayon from this because I'm not sure how we want to do add it. Manifold uses set thresholds to decide whether or not to do parallel execution, and I don't know if rayon's with_min_len() function is the correct thing to use. (I guess it could be further discussed in #14 )

[luisfonsivevo]

With this feature being non-critical and the test suite project coming to a close, I'm going to start being less rigorous with reviewing. Also that screenshot looks convincingly spherical.