getting data from the weather station is now done in _get_data() rather
than measure() (which calls it)

this allows other things to get the data and have added
get_raw_system_data() and get_raw_sensor_data() methods, if the caller
is interested in the raw data

did this to try and find the information about battery levels (which
weren't found) - will try looking in other packets

also:

clearly listed, in get_raw_system_data() and get_raw_sensor_data(),
what the bytes in the raw data mean