Adds latch functionality

[prarobo]

Allows a latch argument to be passed for making the transform publisher latched

[mikepurvis]

LGTM. Mirrors rostopic pub --latch in

ros_comm/tools/rostopic/src/rostopic/__init__.py

Lines 1733 to 1734 in 52567ee

	parser.add_option("-l", '--latch', dest="latch", default=False, action="store_true",
	help="enable latching for -f, -r and piped input. This latches the first message.")

[efernandez]

👍

[mikepurvis]

Looks great, thanks!