- for most field types this is a no-op, but for a field of floatVector
  or doubleVector (eg, vector and solveVector) it will normalise each
  element with divide-by-zero protection.

  More reliable and efficient than dividing a field by the mag of itself
  (even with VSMALL protection).
  Applied to FieldField and GeometricField as well.

  Eg,
      fld.normalise();
  vs.
      fld /= mag(fld) + VSMALL;

ENH: support optional tolerance for vector::normalise

- for cases where tolerances larger than ROOTVSMALL are preferable.
  Not currently available for the field method (a templating question).

ENH: vector::removeCollinear method

- when working with geometries it is frequently necessary to have a
  normal vector without any collinear components. The removeCollinear
  method provides for clearer, compacter code.

  Eg,
      vector edgeNorm = ...;

      const vector edgeDirn = e.unitVec(points());

      edgeNorm.removeCollinear(edgeDirn);
      edgeNorm.normalise();

  vs.
      vector edgeNorm = ...;

      const vector edgeDirn = e.unitVec(points());

      edgeNorm -= edgeDirn*(edgeDirn & edgeNorm);
      edgeNorm /= mag(edgeNorm);