Adding Internal wave test case

As part of the validation of RK3 time stepping scheme, implement the internal wave test case as defined in Marsaleix et al (2008) and Demange et al (2019). It consists in a 2d (x-z) closed domain, initially filled by a constant stratification with a seamount in the middle. An initial sea level slope triggers barotropic waves which generates internal waves on the edges of the seamount.

Marsaleix, P., Auclair, F., Floor, J. W., Herrmann, M. J., Estournel, C., Pairaud, I. and C. Ulses, 2008: Energy conservation issues in sigma-coordinate free-surface ocean models, Ocean Modelling, 20-1, 61-89.

Demange, J., Debreu, L., Marchesiello, P., Lemarié, F., Blayo, E., & Eldred, C., 2019: Stability analysis of split-explicit free surface ocean models: implication of the depth-independent barotropic mode approximation. Journal of Computational Physics, 398, 108875.