new milestones tasks logic in a_todos

[Simon Billinge]

modify a_todos so a todo item can be assigned to a milestone as an option (by giving a milestone uuid fragment). Write tests for everything first, get them reviewed then write the code

• add milestone uuid to cli parser
• write a function in regolith tools module that does the adding (this will be reused in other places)
• if rc.milestone_uuid then trigger logic that adds the uuid of this task to the task list of the given milestone and updates that prum (get that logic from u_milestones...rc.client.update_one())
• news

[dpeters001]

Dom will take this.