Restructure source term implementations.
The implementations of the VolumetricSourceTerm and the LineSourceTerm were almost identical. For this reason the tiny differences have been eliminated while maintaining the functionality. Consequently, the implementation of the LineSourceTerm can be removed.
- Feature description was added to the changelog
- Tests covering your feature were added? - All existing tests pass the CI.
- Any new feature or behavior change was documented? - No change in the behavior for the user.