Improve Dataset model

**Describe the solution you'd like**
Dataset model should have following attributes:
- hours_per_time_step (int, default 1)
- number_of_time_steps (int, default 8760)
- cost_unit (str, default 1e9 Euro)
- length_unit (str, default km)

These attributes must be added to schema and should be validated. Also the test generators should be updated.
