No population -999 code interpreted as negative population when reading GPW

Function get_population_value_for_point in GDE_TOOLS_GPW.py should return zero if the retrieved value is negative in https://gitext.gfz-potsdam.de/dynamicexposure/globaldynamicexposure/gde_calculations_prototype/-/blob/master/GDE_TOOLS_GPW.py#L97.

The version of GPW parsed into an HDF5 file uses -999 to indicate cells in which the original TIFF has no data. These -999 values need to be interpreted as zero population.