Increase Test Coverage

[gugrimm]

Extend test coverage for core modules, e.g., network integration

[jrasko]

The low test coverage of ~45% was due to a wrong coverage reporting, this was fixed in #702. The correct coverage is ~80% which is already pretty decent.
I'd suggest to list functionalities in this issue that need some testing.

[jrasko]

In my opinion, the following test cases may be implemented:

• World & CSV Loader: Custom units
• World & CSV Loader: DSM units
• CSV Loader: Storage units
• Portfolio Strategies: UnitOperatorStrategy & UnitsOperatorEnergyHeuristicCournotStrategy
• Extended Strategies: SupportStrategy & MarkupStrategy
• Interactive Strategies
• Contract Clearing Algorithms
• Outputs (simulation deletion, grid store, flow conversion)