Add filter by Radiosonde Type

Add a dropdown box on the top bar, next to the search box (or perhaps we can make use of the filter box somehow?) to allow filtering by radiosonde type.

Support for this kind of filtering also needs to be added to the Sondehub API. Since the radiosonde 'subtype' (e.g. RS41-SGP) data is not always available, we will have to filter by the 'top level' type (e.g. RS41, DFM, M10...). 
