Dependency on Matplotlib

[henry2004y]

As of now, the plotting functionalities such as arrows rely on Matplotlib. Matplotlib only has fake 3D plots, and there are many difficulties such as creating arrows in 3D. We do not necessarily require PyPlot for plotting; instead, it would be better to have specific plotting utilities for certain packages, similar to the concept of recipe.

I really want to find a better 3D visualization tool as a replacement. Maybe it is also possible to export the field line data to real 3D engines such as ParaView, VisIt, Blender, etc.

[henry2004y]

In a previous implementation of add_arrow:

   if pybuiltin(:isinstance)(line, matplotlib.lines.Line2D)
      xdata, ydata = line.get_data()
   else
      @error "mplot3d does not support true 3D plotting!"
      xdata, ydata, zdata = line.get_data_3d()
   end

This requires PyCall, which causes issues on MacOS. Since this is not working anyway, I decide to remove the check for 3D.