Visualization/Plotting

[dnlbauer]

I am thinking about how different plotting methods could be implemented (overlap matrix, dF/dt,...). On one hand, they all depend on the output of estimators so they could be implemented as additional plugins of type output. On the other hand, I see them as something ceonceptually separate, so maybe it should be a new type of plugins, i.e. visualizers. What do you thing @orbeckst @xiki-tempula ?