Add "color" field to a task, that is exported to JSON, and displayed by the dashboard code

[henricasanova]

As I am developing Pedagogic Modules that display Gantt charts of tasks executions, it's occurring to me that all tasks being the same color in the Gantt chart doesn't make for the best experience for students. This is especially true for DAGs with dependencies. Hovering over all rectangles to find out which task it is is not ideal. Here is a proposal:

• We add a "std::string color" field to wrench::WorkflowTask
• We add setColor()/getColor() methods (with values like "#rrggbb")
• When exporting the JSON for a task, we output the color ("" if none)
• The dashboard code, when plotting the task rectangles, plots it with the default color if color="", or with the requested color

This seems really straightforward to me, but I may be wrong. I've assigned all the relevant people to this issue to that you all can comment if you think it's not feasible or not a good idea