Implements the necessary changes to simplify the exposure database in exposure-lib:
Entity. Now a tile is represented as an entity with
osm_id = NULL.
Implement the checks to not add the same entity twice. I left the unique constraint in but, because all
NULLvalues are distinct, a trigger must check if two times the same tile is being inserted.
Implemented the triggers for inserting and updating.
Closes #17 (closed)