Add support for RTCM & NTRIP GNSS corrections forwarding to ELLIPSE-N & ELLIPSE-D

[rsiryani]

We should update the driver to accept incoming RTCM streams that is directly forwarded to the ELLIPSE-N & ELLIPSE-D. We can use the already existing message: [http://docs.ros.org/en/noetic/api/mavros_msgs/html/msg/RTCM.html] Also look at [https://github.com/dayjaby/ntrip_ros/blob/master/scripts/ntripclient.py]

A dedicated node should be created to:

• connect to a NTRIP client and publish received data to the RTCM message
• listen for NavSatFix messages to publish a GGA message to the NTRIP caster

Thanks to that, ELLIPSE-N/D can provide INS/RTK solutions using one single serial connection and minimal user conf/code.

Please let me know if this could be of interest for some customers / users.