DataSet change breaking report template

[Raymond GA Côté]

The recent DataSet issue (#77) breaks the report templates.
The data column in the report now shows a class __repr__:

DataSet({<pytm.pytm.Data(User ID and SSL Cert.) at 0x105ef5ac0>})

Unless I'm misunderstanding something, I believe adding a __str__ method to the DataSet class (on or about line 191) addresses this issue.

    def __str__(self):
        return ", ".join([d.name for d in self])

Looking forward to making good use of the new DataSet feature.

[Jan Waś]

We were missing a test for reports, thanks for reporting this!