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/test_pandas.py

@@ -21,6 +21,6 @@ def test_pandas(container, name, command):
     LOGGER.info(f"Testing pandas: {name} ...")
     c = container.run(tty=True, command=["start.sh", "python", "-c", command])
     rv = c.wait(timeout=30)
-    assert rv == 0 or rv["StatusCode"] == 0
+    assert rv == 0 or rv["StatusCode"] == 0, f"Command {command} failed"
     logs = c.logs(stdout=True).decode("utf-8")
     LOGGER.debug(logs)