DB - Improve database structure
Improve the database structure without changing the import csv formats.
- For example, timestep unit, from_time and to_time should be should be in the oc or spo table. This will be better for performance when observations table gets bigger.
- Observations should reference oc not spo
- Samples records are not deleted by CASCADE DELETE.
- Fill inn missing values on import to avoid wholes in timeseries