canvas: support comprehension list in Group

[pcaneill]

Bring support for comprehension list in the Group canvas.

It is now possible to write:

Group(
    [ActivityTask(to_string, arg=i) for i in range(0, 2)]
)

[ybastide]

One can currently write

Group(
     *[ActivityTask(to_string, arg=i) for i in range(0, 2)]
)

(the * operator converts the list into a variable length argument tuple; the Group constructor receives this tuple as *activities, which is passed to extend and iterated over. This is more or less well described at Arbitrary Argument Lists and Unpacking Argument Lists in the documentation)

[pcaneill]

lets use *