update to new database repository pattern

controllers/forecast_controller.go

controllers/freezing_dates_controller.go

controllers/gdd_controller.go

controllers/misc_controller.go

controllers/misc_controller_test.go

controllers/nomads_controller.go

persistence/mongodb.go

persistence/repositories/freezing_dates.go

persistence/repositories/gdd.go

persistence/repositories/nomads.go

services/confidence_interval_service.go

services/data_download_service.go

services/data_download_service_test.go

services/forecast_service.go

services/freezing_date_service.go

services/freezing_date_service_test.go

services/gdd_service.go

services/maturity_service.go

services/nomads_service.go