Loads a previously persisted data based on it's unique identifier. 
      Syntax
         
         
            - 
            (
            id< SDMCaching >
            ) 
            
               loadCache
            
            : 
            (
            NSString *
            ) 
            uid_in
         
      
      Parameters
         
         
            - 
               uid_in – 
               the unique identifier for the persisted cache object
            
 
         
       
      Returns
         valid cache object or nil if none found