Change to src/ layout

[nicoddemus]

This is nowdays considered best practice, and in line with our other repos on GitHub.

[prusse-martin]

You will probably also want to update https://github.com/ESSS/qmxgraph/blob/master/docs/conf.py#L23.
But since the documentation build is failing we can postpone this.

[prusse-martin]

I did add 2 items to #37:

• Fix documentation build;
• Check and update examples;

So we can merge this and review the docs later;

[nicoddemus]

You will probably also want to update https://github.com/ESSS/qmxgraph/blob/master/docs/conf.py#L23.
But since the documentation build is failing we can postpone this.

Good point, but went ahead and removed that line: readthedocs has the option to install the project into a virtualenv to build the docs, which removes the need for that workaround. But we still need to fix #37 to get the documentation up and running, as you mentioned.