IS4DVAR preconditioning

[arango]

We are now working on the preconditioning for the IS4DVAR algorithms. Andy Moore coded the spectral preconditioning.

I added two new files (def_lanczos.F and wrt_lanczos.F) to process the Lanczos vectors. We need both Lanczos vectors and Hessian eigenvectors to compute the preconditioning. More updates are coming soon.

Corrected a bug in the observation sensitivity driver. We need to added the spatial convolutions to the Lanczos vectors used to initialize the tangent linear model. The Lanczos vectors are in minimization space (v-space). Many thanks to Andy Moore for finding this problem