Mesh adaption for pseudo-spectral equations
Problem.adapt() calls Equation.adapt() for every equation that is part of the problem.
It should not be too hard to implement (uniform) mesh adaption for PseudospectralEquation: we simply need to add (remove) a padding in the Fourier domain to achieve a finer (more coarse) mesh. The only issue might be estimating the error, but that could also be specific to the equation.