THIS IS BREAKING UPDATE, HENCE THE MAJOR NEW VERSION NUMBER

measure() now returns a WeatherStationData object, rather than storing
the measured data inside the WeatherStation object - this allows
multiple measured datasets to be easily stored

measuring now retries by default, both at the command line and method
level: measure_once() can be used to only do a single measurement (or
tries=1 can be specified)

the timeout and retry algorithm is overhauled to have a maximum number
of tries, rather than a maximum time

various bits of tidying and commenting