Updated adafruit_rfm9x.py to allow optional access to the RadioHead Packet Headers.
The user may optionally set and receive the 4 byte RadioHead packet header.
The default is to ignore the header as in the past.
On transmit, the user may provide a keyword argument: tx_header with a 4-tuple of bytes.
tx_header = (To,From,ID,Flags)
to set the outgoing header.
On receive, the user can request that the packet header be included at the beginning of the packet via a keyword argument: with_header.
If set to True, the header will be included as the first 4 bytes of the packet and the data payload begins at byte 4.
Also, the user may specify a keyword argument: rx_filter to reject any "non-broadcast" packets not addressed as specified. A "broadcast" message contains 0xff as the To address. Broadcast messages are always received.
If rx_filter is set to a value other than 0xff, then incoming packets with the To field not matching the rx_filter will be ignored unless the To field contains 0xff.
If rx_filter is set to 0xff then all packets are accepted.
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://github.com/adafruit/Adafruit_CircuitPython_Bundle).
To use in CPython, `pip install adafruit-circuitpython-rfm9x`.
Read the [docs](http://circuitpython.readthedocs.io/projects/rfm9x/en/latest/) for info on how to use it.