Add custom modem settings

[sharph]

• Adds a new configuration for the modem: SF=12, Cr=4/7, Bw=125kHz, CRC on, Low data rate optimization on
  • Spreading factor 12, 4096 chips/symbol, is the slowest but gets us more link budget: 12.5dB more than what we were using previously according to the RF96 datasheet, p24
  • Coding rate 4/7 was identified as being the best value by @matt-knight in #14
  • Bandwidth left as is. Could use more discussion
  • Low data rate optimization is enabled, as it's needed at larger time scales. This somehow makes the LoRa packets resistant to receiver and transmitter becoming unsynchronized
• Changes default beacon period to 20 minutes, to prepare the network for slower data rates
• Unrelated: adds BATTERY_SAVING constant so we can turn off battery saving modes completely on radios connected to computer or PocketCHIP, for example

This post was helpful in finding the barely documented bit for low data rate optimization mode.

This is just a first pass, which will hopefully generate more discussion in issue #14 about the best configuration to use. Let's hold off on merging pending discussion about bandwidth and if there are differing opinions about the other settings.

[travisgoodspeed]

If we need to do range tests, I could wire up a reflashable node to run nightly builds of this branch in Philly.

[sharph]

@travisgoodspeed That's not a bad idea. If you set it up I'd use it for some comparative testing.

(Sorry for the long delay; work has been busy the past couple weeks.)