problems with non-default spreading factor for rfm9x.

I am once gain going to see if I can make any progress on the non default settings for  the spreading factor, initially reported in:

https://github.com/adafruit/Adafruit_CircuitPython_RFM9x/issues/94
https://github.com/adafruit/Adafruit_CircuitPython_RFM9x/issues/70

I'll post updates here. 

Anyone else is welcome to join in with comments/suggestions.