Converge on timeline notation for ports

[maarten-ic]

The current (ymmsl-python v0.17) way to explicitly attach timelines to ports is to use a +<timeline name> notation, e.g.:

ports:
  f_init: init
  +timeline1:
    o_i: out
    s: in
  o_f: final

The + sign is very magical, and not immediately obvious that this relates to a timeline.
@LourensVeen proposed alternative syntax with a timeline "keyword":

ports:
  f_init: init
  timeline timeline1:
    o_i: out
    s: in
  o_f: final

Since we have no users yet for the timeline functionality, it should be no problem to change the notation.

[LourensVeen]

Part of the reason for this is that it would be consistent with an extension of the settings syntax to allow

settings:
  for component1:
    a: 1
    b: 2
    c: 3

instead of having to type

settings:
  component1.a: 1
  component1.b: 2
  component1.c: 3