Plotting: added class method "get_subplotspec()" for correlation subplots

[lpldmller]

Integration of a Matplotlib API change to plot correlation matrix for versions >=3.6.0: "Subplot.is_first_row(); use ax.get_subplotspec().is_first_row instead. ..." https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.6.0.html#classes-methods-and-attributes