Integrate python graphical examples in the test build

[karlnapf]

Every now and then, we have to re-factor our graphical examples, e.g. #3045

Currently we have no way of checking whether they work.
While we don't want to (Python) unit test these scripts, it would be great if we could at least execute the examples once in our test build. Just like the other python examples

This task is to add the python graphical examples to the test build. This works via creating a CMakeLists.txt file in the appropriate directory, and adding this directory in cmake.

In order to run the graphical examples, the graphical output needs to be re-directed. Otherwise travis and the buildbot will fail. A hack to do this is to append something as

import matplotlib; matplotlib.use('Agg')

to the python example before running it. Google for what it does Here is a hacky way to do this.
@vigsterkr or @lisitsyn might have a better way in their sleves.

[theaverageguy]

I am on it.!

[theaverageguy]

Just checking @karlnapf :

1. shogun/examples/undocumented/python_modular/graphical/ This would contain the CMakeLists.txt .
2. Add this directory in cmake .
3. Appending import matplotlib; matplotlib.use('Agg') to every test python file.

[karlnapf]

something like this. We just want to make sure that all the graphical scripts at least run.
Some care will have to be taken when they go into infinite loops
I suggest googling how to test software with graphical interfaces ... The 'agg' is just a hacky way to do it

[theaverageguy]

I am sending a PR. Please take a look when free @karlnapf . :)
Can I break it into smaller PRs maybe?

[karlnapf]

Please do. Send a PR for a single example first as a prototype. Then we can apply this iteratively to all examples