Fix reading info files with missing tidal model

If there is no tide model selected in the AQG software before starting the measurement, the resulting .info-file is missing the '[tide_model]' section. Without this fix the read_aqg_info() method aborts with a KeyError while trying to read such an .info-file. With this fix a tide dictionary with empty 'location' and 'model' values is assigned in case of a missing tide model.