compute models from instruments in code so that they align

This request removes the automatic calculation of instrument_models and platform_models from the "site" extended resource.  (The automatic traversal of hasModel was wrong anyway -- we want the models of devices associated with the site, not the models associated with the site.)

The removal of the decorator from the comments on these two lines means that the two fields are declared, but not populated by the extended resource framework.  Instead, they will be calculated and assigned inside ObservatoryManagementService, inside get_site_extension.