MAJOR UPDATE. Inversion routine now works with the update of the model executed in rho-vs-vp. That means, seismic &gravity kernels are combined in rhovsvp and then added to the model. This also means that seismic is now added in rhovsvp, unlike in previous inversion routines where it was added in rhomulambda. Also, some minor updates to functions, most notably plot_model which can now handle background values for each of the parameters separately.