[Data assimilation] Interpolation for the base background state and ensemble simulations

[nataraj2]

This PR enhances the capability to do ensemble simulations

1. Reading the base background state on a coarse mesh from a plotfile (Fig. 1).
2. Interpolating the base background state to the simulation mesh (finer mesh) (Fig. 2).
3. Adding spatially correlated perturbations to the base background state.
4. The animations below show the ensemble perturbation for the x velocity for four different ensemble members. The perturbations around the vortex can be seen.