Permit other GPS, RTCM stream via. env vars

First pull request so please let me know if I've missed any best practices.

Added netserial to package scripts.
Optional  sonar.launch uses netserial (socat) as the SonarM8 does not use nmea by default. Space delineated messages were not properly parsed if using nmea_comms.    
Remapping tx to rx was what worked for hydro nmea_comms.  
Netserial for RTCM as RX is binary.