Implement gradient based deformation algorithm

[alexlib]

We do not know the error related to velocity gradients. Apparently, there is an algorithm that could extend our windef

H. T. Huang, H. E. Fiedler & J. J. Wang (1993a). ‘Limitation and improvement of PIV Part I: Limitation of conventional techniques due to deformation of image patterns’, Exp Fluids, 15, 168-174.
H.T. Huang, H. E. Fiedler & J. J. Wang (1993b). ‘Limitation and improvement of PIV Part II: Particle image distortion, a novel technique’ Exp. Fluids, 15, 263-273.

The idea is implemented in DPIVSoft