Allow switching on and off contour plot numerical labels at top level

[misi9170]

Description

The flow visualization tools were hardcoded to plot numerical labels on the flow contour plots. This PR makes that an optional choice at the top level in the visualize_cut_plane() and visualize_heterogeneous_cut_plane() functions of tools/visualization.py, which are passed through to the mid-level function line_contour_cut_plane().

I have set the default value of the new label_contours Boolean to False, as I believe that most users will not want the contour labels shown. However, I've now updated examples 02_visualizations.py and 16_heterogeneous_inflow.py so that label_contours is set to True for those examples. I have left the default value (False) in other examples involving flow plots.

Other notes

• No changes are made to source code
• Output from most examples involving flow contour plots has changed, as the contour labels are now largely gone. The exceptions are 02 and 16, where I have reincluded the contour labels.
• The function visualize_heterogeneous_cut_plane() of tools/visualization.py does not appear to be called anywhere in the floris code or examples, and may be redundant. However, I have not confirmed this and have left the function in place.
• Tests pass.

Examples

Running 02_visualizations.py with label_contours = True produces

while setting label_contours = False produces

[rafmudaf]

Nicely done!