Add damage tables

To enable the loss calculator to store multiple damage assessments per exposure asset, a couple of tables need to be added to the exposure-lib database definition. The tables are:

• DamageAssessment storing the metadata of each damage assessment, being manual through inspection or fully automatic in the loss calculator
• Damage storing the probability of a damage state with the dataset being linked to a damage assessment dataset
• DamageState storing the names of the different damage states
• DamageMethod storing the names of the different damage-assessment methods

A problem remains not perfectly solved: Each damage assessment currently carries a building_id and a quadkey to relate it to either a BuildingAsset or TileAsset. As a consequence, the BuildingAsset and TileAsset tables were unified into an Asset table.

\approve @tara @shinde