Start on code for parsing and storing GPS navigation message data from supplied subframes

Add some printing of subframe 1 to 'parse' for now.
Add class for subframe 4, page 18 (ionosphere and UTC data)

Use it in 'parse'.
Add sign extension to Subframe_data::data() and Subframe::_bits()

I should probably get rid of Subframe_data::data(). Or some other way to avoid this code duplication?
Add GETTER_RAW and GETTER_MOD macros to GPSNav.hh, add exception handling to the later

Still not sure why I'm getting a_f2=0 from Subframe 1 data.
Add documentation to all getter methods