Matplotlib tests + helpers methods

* Maplotlib is an important package. It has now its own test checking that a plot can be generated and exported has an image.
* Some helpers methods added to the `Makefile` mainly permitting to clean containers and images
* Improved assertion messages of some tests

scipy-notebook/test/data/matplotlib_1.py

@@ -0,0 +1,24 @@
+# Matplotlit: Create a simple plot example.
+# Refs: https://matplotlib.org/3.1.1/gallery/lines_bars_and_markers/simple_plot.html
+
+# Optional test with [Matplotlib Jupyter Integration](https://github.com/matplotlib/ipympl)
+# %matplotlib widget
+import matplotlib
+import matplotlib.pyplot as plt
+import numpy as np
+import os
+
+# Data for plotting
+t = np.arange(0.0, 2.0, 0.01)
+s = 1 + np.sin(2 * np.pi * t)
+
+fig, ax = plt.subplots()
+ax.plot(t, s)
+
+ax.set(xlabel='time (s)', ylabel='voltage (mV)',
+       title='About as simple as it gets, folks')
+ax.grid()
+# Note that the test can be run headless by checking if an image is produced
+file_path = os.path.join("/tmp", "test.png")
+fig.savefig(file_path)
+print(f"File {file_path} saved")