add station_type parameter to load data from join

For data downloading from join, it would be great to set the station_type to look for. In @kleinert1's current implementation, this is done inside the code, but leads to the download of many later unused stations. If this option can be set, only used stations are downloaded.

• implement param in join query
• parse param through all modules
• create filename with station type if set
• can load filename with station type if set
• check in meta data if local available station fulfil station_type criteria