[44net] NPR (New Packet Radio) : new firmware with 56kBaud and 120kBaud

Hello.

I have updated the firmware of my NPR (New Packet Radio) project with 2 expected features, mainly for America:  * New modulations (11, 12, 20, 21) for lower datarates, lower bandwidth. Symbol rates: 56kS/s (complies with FCC) and 120kS/s. You now have the choice between 9 modulation parameters, in order to adapt to lots of situations and needs.  * Frequency range extended to 420-450MHz instead of previously 430-440MHz

I have updated all the documentation accordingly. https://hackaday.io/project/164092-npr-new-packet-radio/

Several people here expected these features. I hope you will enjoy it!

73, Guillaume F4HDK