MatplotlibDeprecationWarning on gca() in the FlowDirector and FlowAccumulator nbks

[sebastien-lenard]

The function surf_plot() in the three notebooks compare_FlowDirectors, the_FlowAccumulator, the_FlowDirectors, instantiate the axes of a figure by the call
ax = fig.gca(projection='3d').

This call raises the following warning:
/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel_launcher.py:26: MatplotlibDeprecationWarning: Calling gca() with keyword arguments was deprecated in Matplotlib 3.4. Starting two minor releases later, gca() will take no keyword arguments. The gca() function should only be used to get the current axes, or if no axes exist, create new axes with default keyword arguments. To create a new axes with non-default arguments, use plt.axes() or plt.subplot().