select matplotlib backend only when needed
Previously, in order to not to fix the matplotlib backend, we required users to import matplotlib.pyplot before calling any Kwant plotting functions. This did not have the desired effect, since we did import `matplotlib.backends` and that also fixes the backend. Now, both backends and pyplot are imported at the last possible moment and a warning is emitted if this fixes the backend.
Showing with 35 additions and 20 deletions