Migrate download_model_RDS into useeior

https://github.com/USEPA/USEEIO-State/blob/baaf100e78ad440694a1d0c439b7070244029c7c/R/utils.R#L18

This would be useful for users to get models. 
The directory on the Data Commons would need to be made dynamic or searches across applicable directories would need to be added.