- signedDistance() method is like distance() but retains
the positive/negative sign for the side of the plane.
- the sign() method returns the sign as -1,0,+1 integer for
classification purposes where it is important to distinguish between
a zero value and a positive value (eg, for cutting). Optional
tolerance can be supplied to round for zero.
- refactor and inlined simple and frequently used methods.
- add boundBox faceCentre() method, which can be useful for creating
clipping planes from a bounding box.
Relocated treeBoundBox faceNormals to boundBox since they apply
equally there - the meaning of the faces (x-min, x-max, etc)
is the same, even if the point addressing for the faces differs.
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