Add an example for the preprocessor.py

An example needs to be added to the repository to show how to use the preprocessor.py.
This example will also better show how an OpenQuake-compatible exposure model CSV file can be imported into database and then also how such a database can be exported to an expanded CSV exposure model file (Here expanded means having the tile_id, tile_geometry, building_id and building_geometry added to the input OpenQuake-compatible CSV exposure model file).

Also add the An example of the database exposure model is available at .... to the manual.md under ### Using import command and same for ### Using export command.