SRM: stop setting default note when writing

old SRM file formats didn't have explicit space for storing the
temperature. Instead srmwin put the average temperature into the file's
note. This isn't needed with SRM7 or later files.

To avoid confustion (a note showing up without being set by the user) we
stop replacing an empty note with the average temperature.
SRM: handle invalid elevation

apparently elevations above 65000 are used by SRM to indicate "no value".

Although bad user calibration of pressure sensor may cause such values,
this is too rare. So, we're taking everything above 65000 as "no value".

This way we're loosing the chance to read + fix the elevation data for
such bad miscalibrations.