Major update with lots of changes:
- Cleaned up code-base
- Moved a lot functionality into extensions, i.e. `obs.GroundwaterObs.geo.get_lat_lon()`. Available extensions are: `geo`, `gwobs` (groundwater observations), `stats` and `plots`
- New way of dealing with metadata that changes over time.
- Improved IO, more uniform approach for all the different supported data types.
- And many smaller improvements...