F-xy eigenimage noise suppression

F-xy eigenimage filtering works by replacing constant-frequency slices with the sum of their first few weighted eigenimages, and can be used to remove random noise from stacked 3D seismic volumes. It performs equally well on flat or dipping events, and has no effect on noiseless data when the number of dips is no more than the number of summed eigenimages. It is also independent of many x- and y-consistent effects such as reordering, statics, and filtering. One consequence is that it tends to do a good job along the boundaries of the 3D grid. Execution time is comparable to f-xy prediction filtering, but can be greatly reduced using approximations based on Lanczos bidiagonalization.