Better tests for problem classes

[pancetta]

• [ ] Advection_ND_FD: Dirichlet and higher dimensions
• [ ] AllenCahn_MPIFFT: IMEX and rand_circle (do we need IMEX here?)
• [ ] AllenCahn_Temp_MPIFFT: rand_circle (what about IMEX here?)
• [ ] HarmonicOscillator: init and damping
• [ ] HeatEquation_1D_FEniCS_weak_forced: fenics_heat_weak_fullyimplicit not tested whatsoever
• [ ] HeatEquation_ND_FD: Gaussian and higher dimensions
• [ ] AcousticAdvection_1D_FD_imex: all those helper classes (ugh!)
• [x] AdvectionDiffusionEquation_1D_FFT : the whole module