change default hold_after_set to True (#835)

qcodes/instrument_drivers/oxford/mercuryiPS.py

@@ -82,7 +82,7 @@ def __init__(self, name, address=None, port=7020, axes=None, **kwargs):
         self.add_parameter('hold_after_set',
                            get_cmd=None, set_cmd=None,
                            vals=Bool(),
-                           initial_value=False,
+                           initial_value=True,
                            docstring='Should the driver block while waiting for the Magnet power supply '
                                      'to go into hold mode.'
                            )