Make the message __repr__ for python look nicer.

Before this patch, publishing a message with "ros2 topic pub"
would print something like:

publishing #5: my_msgs.msg.my_msg(axes=array('f', [0.0]))

While that is OK, it is kind of ugly.  This patch hides the
typecode and the "array" so that the output looks like:

publishing #5: my_msgs.msg.my_msg(axes=[0.0])

Which is also how it looked in Crystal.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

rosidl_generator_py/resource/_msg.py.em

@@ -334,7 +334,18 @@ if isinstance(type_, AbstractNestedType):
         typename = self.__class__.__module__.split('.')
         typename.pop()
         typename.append(self.__class__.__name__)
+@[if 'import array' in imports]@
+        args = []
+        for s in self.__slots__:
+            field = getattr(self, s, None)
+            if type(field) == array.array:
+                t = repr(field.tolist())
+            else:
+                t = repr(field)
+            args.append(s[1:] + '=' + t)
+@[else]@
         args = [s[1:] + '=' + repr(getattr(self, s, None)) for s in self.__slots__]
+@[end if]@
         return '%s(%s)' % ('.'.join(typename), ', '.join(args))
 
     def __eq__(self, other):