Fix msg printer for arrays

[v4hn]

I put this patch here for visibility. @rhaschke pushed it to the ROS-O fork last year.

Robert: Maybe you can comment with an example for completeness?
I believe this is related to message parsing in the context of MTC properties, right?

[v4hn]

Related to https://github.com/ros-o/roscpp_core/commits/obese-devel/ (ros-o/roscpp_core@bdb975d, ros-o/roscpp_core@fe49887, ros-o/roscpp_core@cae00b4)

[rhaschke]

Printing messages via the ostream operator << doesn't yield valid yaml in roscpp (while it does in rospy). For example, given these simple messages:

main.msg:

uint32[] ints
sub[] subs

sub.msg:

uint32  i
float32 f

yields the following output for std::cout << msg in ROS Noetic:

  ints[]
    ints[0]: 1
    ints[1]: 2
    ints[2]: 3
  subs[]
    subs[0]: 
      i: 0
      f: 0
    subs[1]: 
      i: 0
      f: 0

while it should look like this (to be compatible with rospy/yaml and realized by this PR in ROS One):

  ints: [1, 2, 3]
  subs: 
    -
      i: 0
      f: 0
    -
      i: 0
      f: 0

I remember having seen even worse formatting, but I can't reproduce that quickly.