Make Dataset a Context Manager. (#68)

Dataset can now be used in a with statement.

ismrmrd/hdf5.py

@@ -158,7 +158,13 @@ def __del__(self):
             self.close()
         except:
             pass
-
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.close()
+
     @property
     def _dataset(self):
         if self._dataset_name not in self._file:
@@ -167,7 +173,7 @@ def _dataset(self):
 
     def list(self):
         return self._dataset.keys()
-    
+
     def close(self):
         self._file.close()