fix merge failed when launch merger faster than laser

[Tacha-S]

Related Issues or PRs
#5 #16 #17 (maybe #32)

#16 and #17 have a problem.
These PRs never merge when laserscan_topics contained one or more unknown topics.
To prevent it, when triggered scanCallback by one of subscribers, clouds_modified that the topic has no publisher set to true.

I hope this package works better regardless of launch order and makes this package more convenience.

[JackFrost67]

i've tested, but it doesn't work, thank you for the pull, I've made some changes and now there is no problem with the launch of the merger.