station: handle attributes of longitude/latitude
station: changes to float derived types, thanks @markcwill for the
prototype
station: redo float derived classes (to be able to override __init__)
station: fix xml comparison in tests (+fix mapping of uncertainties in
read/write)
station: correctly handle several more float based data types
station: write channels in output
station: make sure floats are output to string to full accuracy
station: refactor float derived classes, better handling for
fixed/settable unit attribute, also implement wo missing tags regarding
sampling rate in channel
station: correctly write some more tags
station: move new float derived types to core.util
Merge pull request #688 from megies/obspy.station_lonlat

station: handle float derived types