Plot 2D improvement for models with complex topographies

[rdno]

Hi Bryant,

This patch adds plot_2d_image function and uses it in seisflows.tools.specfem.Model

It adds NaN values to the top edge of the model before interpolation to the regular grid. In order to fix the representation issue with complex topographies. I have also tested with flat topography, it didn't cause problems.

I couldn't it make it work tricontourf. Feel free to explore similar fix for it. plot_2d_image might be slower than plot_2d_contour since it uses imshow which requires a interpolated regular grid.

Comparison image for the simple_topography_and_also_a_simple_fluid_layer specfem2D model.

from seisflows.tools.specfem import Model
m = Model("DATA")
m.plot2d(parameter="vs")

[bch0w]

Thanks Ridvan! The new figure looks great