Update explorer and add more functionality

[gutzbenj]

wetterdienst explorer needs an update:

• [x] Enable select station by click on an icon on map (requires ipyleaflet)
• [x] Zoom map to selected station
• [x] add unit to y axis
• [x] add new preview picture based on current layout
• [ ] Fix x-axis labels in graph figure
• [ ] Implement different figure types for different data types
• [ ] Provide more information for weather station (location, avail.) on the front-end
• [ ] Display extremes of actual visualisation
• [ ] Support overlays