Use a macro to define all of the operator<<() methods since all they do is use std::to_string()
Calculate and display UTC time in the Measurement_time message handler

This requires us to pull out the leap second offset from page 18 of subframe 4. It starts as 16 seconds, which is the current value.
Add data() template method to Subframe_data message class for extracting an arbitrary range of bits

Not the most efficient implementation, but it seems to work for now.