Rename `DamageMethod` table to `AssessmentSource`

The DamageMethod table currently holds strings for different damage assessment methods. However, it would be beneficial to have unique identifier for damage assessment that belong together, being created in a run of the loss-calculator or in a campaign of manual damage assessment per building. Therefore, the table needs to be renamed into AssessmentSource and contain two fields, one for describing the method (e.g. loss-calculator, manual assessment) and one for naming the set of assessments.