Include PropertyMap.a property

This provides convienent access to the underlying array values, such as:

   p.a = random(len(p.a))

instead of:

   p.get_array()[:] = random(len(p.get_array()))

(the latter is still supported)

src/graph_tool/core.py

@@ -182,7 +182,7 @@ class PropertyMap(object):
         return self.__map.value_type()
 
     def get_array(self):
-        """Get an array with property values
+        """Get an array with property values.
 
         .. WARNING::
 
@@ -201,6 +201,17 @@ class PropertyMap(object):
             n = 1
         return self.__map.get_array(n)
 
+    def __get_array(self):
+        return self.get_array()[:]
+
+    def __set_array(self, v):
+        self.get_array()[:] = v
+
+    a = property(__get_array, __set_array,
+                 doc=r"""Shortcut to the :meth:`~PropertyMap.get_array` method
+                 as a property. A view to the array is returned, instead of the
+                 array, for convenience.""")
+
     def is_writable(self):
         """Return True if the property is writable."""
         return self.__map.is_writable()