Extend the examples in getting_started.rst and run them as unit tests

[tovrstra]

The examples in getting_started.rst are a bit limited and could be extended (in line with the manuscript). They should also be tested in the unit tests to make sure they keep working. This can be accomplished by writing them as python scripts in the iodata/test/data directory. These files can be easily accessed by the unit tests and included in the documentation with the literalinclude directive.

• Put examples in Python scripts and run them as unit tests. See Test documentation build in pull requests #333
• Add a few more examples, e.g. from manuscript

[tovrstra]

Adding more examples is not critical for 1.0.0, so I'm removing this issue from the milestone.