Appending value to existing metadata

[jahagirdar]

This is an extension of #379

How do I append to an existing field instead of replacing it?

Background:
I have a script which exports a taskjuggler list of tasks from taskwarrior.
to get around the multiple dependency issue in taskwiki, I have

created an UDA `tj3depends` and
added a function get_uuid() to taskwiki's SelectedTask class which sets a vim variable `taskwiki_tj3uuid`
Now I need to define an append_dependency function which will append the previously retrieved uuid to the tj3depends of current task.

Looking at modify, function I guess I need to

1. retrieve the current value of the tj3depends field as a list.
2. Update it i.e. append new uuid if not present.
3. Save the task i.e. run util.tw_execute_safely

I am not sure about the exact command to use for the first step.

From tasklib raw commands what I am trying to do is

t=tw.get_task(uuid=selectedTaskID) 
dep=t["tj3depend"]
if taskwiki_tj3uuid not in dep.split(' '):
  dep=dep+f" {taskwiki_tj3uuid}"
  t["tj3depend"]=dep
  t.save()