Add an example for the preprocessor.py
An example needs to be added to the repository to show how to use the
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
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
### Using import command and same for
### Using export command.