Contour plot in PEST coordinates for the radial B field perturbation

[pazathoth]

As discussed over the thesis presentation, it would be interesting to plot the magnetic field perturbation not only in (R, Z) coordinates as in scripts/tri_plots.py, but also in (psi, theta), in order to visualize the connection to poloidal modes. Once MEPHIT is up and running, this would be a good first task to get familiar with the code.