implement changes per the errata in https://semtech.my.salesforce.com/sfc/p/#E0000000JelG/a/2R000000HSPv/sqi9xX0gs6hgzl2LoPwCK0TS9GDPlMwsXmcNzJCMHjw

added a new kwarg "agc" which can be set to enable the automatic Gain Control - Default is False to disable so there is no change to existing behavior.
The only change to existing behavior is to set bit 7 of Register 0x31 per the errata. For all BW except 500K it is now set to 0.

To address some of the items in #51 several new attributes were added:
auto_agc -- set via agc kwarg or can be set manually after init
low_datarate_optimize - defaults to 0 can be set if desired
lna_boost_hf -- defaults to 0 (off) nay be set to 3 to enable
auto_ifon -- controls bit 7 of register 0x31 (used internally when setting BW)
detection_optimize - (use internally when setting SF)

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip3 install adafruit-circuitpython-rfm9x.

Read the docs for info on how to use it.