Changing `damage_calculator.py` to match new result database table name changes

By adding loss computation to the losscalculator there was a need to change the database structure (the database in which the damage and loss results are written to).

one of the steps towards this change was to Rename DamageAssessment table of the result database to Assessment(exposure-lib#36 (closed)), because this table can also be used for losses and not only for damage.

Now the damage_calculator.py needs to adjust to this change and write to the table named Assessment and not DamageAssessment. I will address this problem in this issue.