Make BaseRunner an abstract base class

## ([original issue on GitLab](https://gitlab.kwant-project.org/qt/adaptive/issues/107))

_opened by Jorn Hoofwijk ([@Jorn](https://gitlab.kwant-project.org/Jorn)) at 2018-09-24T18:39:28.061Z_

This actually isn't really important, but my linter was complaining... It would be just so that no-one can initiate a `BaseRunner`