Code coverage and example move

[quaquel]

With the move of examples to mesa.examples, we currently don't meet the code coverage target.

I guess the best solution is to explicitly add a test_examples.py to mesa.tests, which tests all examples. Curious what other think.

[Corvince]

We already have such tests! But I guess they stopped working since the examples are now in basic and advanced folders? But now that the models are importable the logic can probably be vastly simplified.

[quaquel]

the current test_examples in mesa.tests is currently explicitly skipped. I'll rewrite the tests given the new structure and make them work again hopefully somewhere this weekend.