Add _clean methods to shr_strdata_mod to cleanup memory usage

In CTSM we use streams for files with a single time instance. Hence these files can be used at initialization and then closed and memory deallocated afterwards. We have a quarter degree dataset that has 4 million points and as such it would be helpful to deallocate all the memory for it. There are a couple other cases where closing these files would help with our memory profile, so this would be worth doing.