Handling of calibration factors when reference values are zero
In general, the reference values for population and structural value are taken from the aggregated exposure models and thus they are always larger than zero. In the case of countries/territories for which no aggregated exposure models exists, we created simplified models (see https://git.gfz-potsdam.de/globaldynamicexposure/collaboration/-/issues/584). In these models the total structural value is set to 0, causing a 'division by zero' exception when computing the calibration factor. A solution is to set the calibration factor to N/A in case the reference values are 0.