Fixed the BiQuadraticQuad interpolator

The original ordering switched x-coords with y-coords, generating
incorrect coordinates for the interpolated point.

Change-Id: I84fa6b399746523c5a7478f693e08e3253a55b48
Added a test for biquadratic quad interpolation

This is based on the code in https://github.com/Kitware/VTK/pull/5
submitted by Antonio Cervone.

Change-Id: I9416f7d9d4796d804ef327c11ebfe77e2bf58e1a
Make consistent with vtkBiQuadraticQuad

Thanks to John Platt for pointing out this inconsistency.

Change-Id: Id4f1e251014bfdb3babc79af1e083825bce6db07
Merge topic 'biquad-interpolator' into master

e768cfb1 Make consistent with vtkBiQuadraticQuad
0b0b7b9e Added a test for biquadratic quad interpolation
588fc24f Fixed the BiQuadraticQuad interpolator