I registered a "slow" mark in pytest.ini and
skip tests with this tag by default.

The test added runs an inversion for a week of data, with 5000
iterations.

I'm not sure how useful it is now, although if this test runs quickly,
then there is probably something wrong with the model.

I changed it temporarily to help debug something and forgot
to change it back.

Now these are created automatically by conftest.

Checks are made to avoid overwriting any files already
present in these directories.

In the future, these files will be managed by something like
the object store.