Moved model-specific parameters from Configuration to ExposureModelESRM20
In !4 (comment 69192) we identified that we were moving forward in an inconsistent way regarding what was defined in the configuration file and became part of the Configuration class and what was being hard-coded within the ExposureModelESRM20 class. We have opted to hard-code the parameters that are properties of ESRM20 to the ExposureModelESRM20 class and thus eliminate the need for these parameters to be defined in the configuration file. This is because the user would need to know these parameters for ESRM20 to be able to run the code, i.e. there was no "free choice" of parameters possible. In other words, by indicating "esrm20" as the exposure_format in the configuration file, these parameters are implicit.
Due to this change, retrieve_exposure_entities is now being called from ExposureModelESRM20.__init__ and not AggregatedExposureModel.__init__, because it needs occupancy_cases to exist.