Define interface for plotengines
During the course of implementing the fallback and maybe a second plot library, it the necessary changes / interactions between plotID and the engine should be collected, organized and structured so we can define an interface to enable users to implement plot libraries on their own.
This should also include a way to detect whether the plot was made by a supported engine - so we need a list / build a list of supported engines. (compare #35 )
We can also think about whether all supported engines should be installed along with plotID - or how we handle calls to not installed engines. --> For unittesting we need all engines installed.