Thorough testing and API support for plotters and plotter parameters

As of today, the concept of a plotter is implemented in beat.core. It would be good to extend its implementation in the core so that:

1. Plotters can be downloaded/uploaded/edited using the web API and via the command-line tool
2. A command-line utility is made available to plot a given experiment output
3. Plotter parameters are instantiated in the core, so that we can also apply them while locally plotting

This should be done after the FG tutorial.