improve persistent parameters

A value given in config overrides values read from the persistent data file.
To let the loaded parameter have precedence, configure a default only.
The write_<param> method of a persistent parameter is now always called
on startup.

- add tests for persistent behaviour
+ simplify Modules.writeInitParams: remove started_callback argument

Change-Id: I08b49de52e9d9a2ed0918018eb2fe538141a4f5e

frappy_psi/trinamic.py

@@ -270,7 +270,7 @@ class Motor(PersistentMixin, HasIO, Drivable):
         elif not self._loading:  # just powered up
             try:
                 self._loading = True
-                # get persistent values
+                # get persistent values and write to HW
                 writeDict = self.loadParameters()
             finally:
                 self._loading = False