Fix `ModuleNotFoundError` for `SA_post_hoc_analysis.plot_corr_heatmap(...)` function.

[tobeycarman]

In [1]: import mads_calibration.SA_post_hoc_analysis as sap
In [37]: sap.plot_corr_heatmap(sap.calc_correlation(sample_matrix, results))
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[37], line 1
----> 1 sap.plot_corr_heatmap(sap.calc_correlation(sample_matrix, results))

File /work/mads_calibration/SA_post_hoc_analysis.py:311, in plot_corr_heatmap(df_corr)
    304 def plot_corr_heatmap(df_corr):
    305   '''
    306   ??? Write something...
    307 
    308   .. image:: /images/SA_post_hoc_analysis/correlation_heatmap.png
    309 
    310   '''
--> 311   import seaborn
    313   plt.figure(figsize=(15,10))
    314   seaborn.heatmap(df_corr, cmap="YlGnBu", annot=True, fmt=".2f")

ModuleNotFoundError: No module named 'seaborn'

Probably better to import this at the module level anyways...