Plot arbitrary field (as option in 2D and 3D)

[Leguark]

Is your feature request related to a problem? Please describe. Adding an example passing arbitrary scalar fields to the plot 2d and plot 3d

[ ] If the computation of the gradient of the scalar field -used for the surfaces interpolation- becomes an standard gempy output before this issue is done, we should add the option to directly plotting the gradient too

[AndrewAnnex]

I just ran across a use case where it would be very useful for me to have access the the gradient of the scalar field, specifically for custom grids

[Leguark]

GemPy v3 has a better implementation of interpolating the gradients. To date there is no API but you can find some tests in gempy_engine