Fix installation of package

After installing the package and its dependencies and starting the example with:

pip install .
losscalculator -f example/example_cell/data/fragilities -t example/example_cell/data/taxonomy_mapping.csv -g example/example_cell/data/ground_motion_values.csv -c example/example_cell/data/cell_ids.csv -e example/example_cell/data/exposure.csv -p example/example_cell/data/polygons.csv -r example/example_cell/result.csv -o

the following error is thrown:

  File "/home/user/Code/gde/losscalculator/venv/bin/losscalculator", line 5, in <module>
    from losscalculator.damage_calculator import main
ModuleNotFoundError: No module named 'losscalculator'

\approve @ds @fd