Adding Nektar++ driftwave 2D Hasegawa-Wakatani example model

[matt-graham]

Adds a new test model using Nektar++ 'driftwave' solver for two-dimensional Hasegawa-Wakatani equations.

The deterministic components dynamics solve two-dimensional Hasegawa-Wakatani equations with spatially correlated additive state noise simulated by solving a Helmholtz equation driven by a Gaussian white noise process. System is simulated on a rectangular spatial domain with periodic boundary conditions, and a regular quadrilaterial mesh using Nektar++ spectral
element method implementation.

Currently for simplicity even though the Nektar++ solvers can be compiled with MPI support, this assumes we just run the solver instances for each particle in a single rank. Could be worth investigating using MPI_Comm_spawn or another approach to allow using MPI at both model and particle filter level.

[matt-graham]

Thinking about it, this probably should live in https://github.com/Team-RADDISH/ParticleDA-UseCases instead