Should support different bearer profiles

[AlanBell]

Several commands contain a 1 e.g. 'AT+SAPBR=3,1,"APN","{}"' that 1 is the bearer profile identifier, and there can be several, perhaps pointing to different APNs. The UNV SIM868 module (which more or less works with this code) appears to have a broken bearer profile 1, which always reverts the APN to CMNET. The SIM800 series AT command manual seems to indicate valid bearer profile numbers are 1-3 but I guess other hardware could have more or fewer. I would suggest an init parameter bearer=1 that should be inserted in relevant commands. I will probably do a pull request for this in the next weeks.