Return Pareto plot as Bokeh JSON

The frontend will support rendering interactive plots based on JSON exported by Bokeh on the server.

In https://github.com/BoostV/process-optimizer-api/blob/4a6f75fb11c44de7070a1f791d3789330ddae76f/optimizerapi/optimizer.py#L218 the `plot_Pareto` method should be replaced with a method producing Bokeh JSON.

Ideally plot output should be completely controlled by the feature implemented in #46, however, in the short run we can make do with just having the Pareto plot as JSON.