Root traits saved in JSON files

[eberrigan]

• Currently per frame traits are summarized in trait_pipelines.compute_plant_traits(). The pre-summary traits which are per root are of interest.
• There should be an option to save per root traits in compute_plant_traits() similarly to compute_multiple_dicots_traits().
• Similarly, then we can provide an option to get aggregated trait statistics over all roots over all frames when frame-specific data is not required.