Add sonde tests

[tmieslinger]

In PR #187 tests are removed that used to test the A-files and Sonde object generation. Instead, it would be nice to have tests that work with actual example data and create a Sonde object of known state.

It would be good to implement tests for:

• processing a regular sonde from D-file without A-file -> launch_time should be NaT, but sonde_id is known
• processing a regular sonde from D-file with A-file -> check launch_time and sonde_id ...
• processing a "minisonde"