surface_levelset: Fix a nasty bug

Someone snuck a "0*" into the code in the subtlest way possible in
order to (1) slow down the code and (2) patch around an actual bug
in the bittlest way possible.  I'm not sure who.
RawField: Fix memory bug in from_python conversion

from_python<RawField<...>> must return a Field, not a RawField.
If we return a RawField, any newly created arrays will be deallocated
immediately, causing subtle memory bugs.
TriangleTopology: Speed up is_manifold_with_boundary from O(n) to O(boundary)
Implement approximate mesh offsets