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/lib/statemachine.py

@@ -178,7 +178,9 @@ class StateMachine:
                 if self.stop_exc:
                     raise self.stop_exc
             except Exception as e:
-                self.log.info('%r raised in state %r', e, self.status_string)
+                # Stop and Restart are not unusual -> no warning
+                log = self.log.debug if isinstance(e, Stop) else self.log.warning
+                log('%r raised in state %r', e, self.status_string)
                 self.last_error = e
                 ret = self.cleanup(self)
                 self.log.debug('cleanup %r %r %r', self.cleanup, self.last_error, ret)