[Data assimilation] Interpolation for the base background state

[nataraj2]

This PR enhances the capability to do ensemble simulations with the routines needed for

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.