support write_ method on readonly param and more

- write method may be used internally on a readonly parameter
+ add IDLE, WARN, BUSY and ERROR to secop.core
+ secop.datatype.EnumType: allow 'self' as member name
+ secop.lib.statemachine: log Restart and Stop exceptions only on debug level
+ secop_psi.ccu4.CCU4: explicit conversion to float
+ secop.proxy: remove superfluos and erroneous make_secop_error

Change-Id: I2f13d31ceacd2bde65eab64f8eae4225556c18f5
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/27963
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/persistent.py

@@ -75,7 +75,7 @@ class PersistentMixin(HasAccessibles):
         self.initData = {}
         for pname in self.parameters:
             pobj = self.parameters[pname]
-            if not pobj.readonly and getattr(pobj, 'persistent', 0):
+            if hasattr(self, 'write_' + pname) and getattr(pobj, 'persistent', 0):
                 self.initData[pname] = pobj.value
                 if pobj.persistent == 'auto':
                     def cb(value, m=self):