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.