improve parameter initialisation

- make 'value' a Parameter property instead of an attribute
- use 'value' instead of 'default' property for setting
  the initial value in the config file
- removal of initwrite parameter property

this change is the basis of a better implementation
for change 30041 (PersistentParam property 'override_cfg')

Change-Id: I2b82bdd54c2dacb87dcd2b3472004d2f0a730cf0

test/test_poller.py

@@ -132,6 +132,7 @@ def test_poll(ncycles, pollinterval, slowinterval, mspan, pspan, monkeypatch):
     m.pollinterval = pollinterval
     m.slowInterval = slowinterval
     m.run(ncycles)
+    print(getattr(m.parameters['param4'], 'stat', None))
     assert not hasattr(m.parameters['param4'], 'stat')
     for pname in ['value', 'status']:
         pobj = m.parameters[pname]