- drop python 3.5 compatibility, require at least 3.6
- fix exception when trying to show plots if loading a madx file that includes
a ``SELECT, flag=TWISS`` command
- fix weird matplotlib offset behaviour when showing a nearly constant quantity
(by plotting an invisible horizontal line at y=0)
- add menuitem to redo twiss and refresh plot
- add menuitem to set number of interpolation points
- add config entry for number of interpolation points
- fix some warnings/errors in documentation
- replace QUndoStack by our own implementation to simplify using Model in
non-GUI contexts
- remove QUndoView for now (limitation due to replacing QUndoStack)
- simplify setup.py using static metadata and rework travis scripts