Return Pareto plot as Bokeh JSON

[langdal]

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

In

process-optimizer-api/optimizerapi/optimizer.py

Line 218 in 4a6f75f

plot_Pareto(optimizer)

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.

[langdal]

The frontend now supports rendering either PNG or JSON plots. It will dynamically determine the correct rendering based on the contents of the plot field.

Pending this PR: BoostV/process-optimizer-frontend#198