ROS Adds ability to publish an rtcm_msgs Message instead of a mavros_msgs RTCM

[robbiefish]

• Adds launch file parameter rtcm_message_package which can be specified as one of the following:
  • mavros_msgs to publish a mavros_msgs/RTCM
  • rtcm_msgs to publish a rtcm_msgs/Message

[PaulBouchier]

Thank you for implementing this new feature. I tested branch feature/ros_rtcm_msgs to make sure it all worked properly, and it did. (I passed in the arg rtcm_message_package set to rtcm_msgs, and my F9P acquired RTK Fixed mode right away using my local rtk2go ntrip server.

I want to recommend an additional change: document the new launch arguments in README.md. Perhaps some explanatory text that identifies the two different message packages supported would be appropriate. Separately, I noticed the new args ntrip_version and debug are not documented in the README either (but maybe they're documented on a different branch).

This PR fixes my issue #10 and I think it should be merged after the README fix proposed, and the issue closed. Thanks again for the enhancement - you guys rock!

[robbiefish]

@PaulBouchier I am glad this worked well for you.

The readme is pretty far behind on documentation. I will make a separate PR to update the readme with all the new launch params and try to be better about adding the documentation in the PR from now on