Re-work matplotlib/seaborn visualization

[jorisvandenbossche]

Idea to switch the order of matplitlib/seaborn:

• We first see seaborn as the high-level plotting library, and only afterwards learn about matploltib being the underlying library for both pandas plot and seaborn
• Matplotlib is then the library with which you can further tweak the resulting plot from pandas/seaborn, or set up custom subplots if needed
• We would then need to remove all direct matplotlib interaction from the early pandas notebooks and first case study? (e.g. fix, ax = plt.subplots() ... where this is not strictly needed)