ENH: additional methods and improvements to plane
- 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.