add close sas session function

`saspy.SASsession` has a method `endsas` to close the SAS session. by default it is not executed in __del__ method, and this can lead to remote SAS session not closed. in saspy, I think the author want the users to use context like
```
with saspy.SASsession() as session:
session.do_something()
```

since we are storing the SASsessions somewhere, I think we should call the endsas method when R exits. In addition, it is also possible for users to manage these sessions too