Reading North America Call Signs

[dpwe]

In North America, radio stations have 4-character call signs (e.g. "WNYC" or "KQED") which are encoded into the Program Identification (PI) code algorithmically (see section D.6 of the US RDBS standard, http://www.interactive-radio-system.com/docs/rbds1998.pdf). This replaces using the PS of GT0A to carry the station name.

I've adapted the "scan" and "spectrum" commands to obtain and the call letters from the PI instead of scanning for GT0A: https://github.com/dpwe/RdSpi/tree/na_pi . However, I haven't preserved the GT0A.PS-reading code; we need some additional work to support both North American and European standards in the same code.