add __enter__ and __exit__ methods for with statements (#121)

Lib/dataset.py

@@ -1204,6 +1204,10 @@ def __init__(self, path, mode, hostObj=None, mpiBarrier=False):
             self.close()
             raise
 
+    def __enter__(self):
+        return self;
+    def __exit__(self, type, value, traceback):
+        self.close()
     # setattr writes external global attributes to the file
     def __setattr__(self, name, value):
         self.__dict__[name] = value  # attributes kept in sync w/file