reimplement Database.add_property()

the value type is determined automatically unless specified
set database version back to 1

and revert migrate_db back to before these database updates occurred.
Since these changes only add to, or delete from, the existing database
without modifying the existing tables the database does not need to be
manually updated (migrated).  sqlalchemy will add the new table
SystemParameters automatically, and ignore the (now deleted) table
Distance, if it already exists.