Optimise WaveDiff to address Centroid problem

[jeipollack]

During PSF model optimisation, the centroids between two images: reconstructed and actual must be the same. Else, the residual errors will be large even though the model is a good representation.

To enable better centroiding requires reintroducing a PSF model called TF_physical_poly_field which is a polychromatic PSF model with a physical layer.

[jeipollack]

Open a new issue on integration of the physical layer PSF model.