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/core.py

@@ -31,7 +31,7 @@ from secop.datatypes import ArrayOf, BLOBType, BoolType, EnumType, \
 from secop.iohandler import IOHandler, IOHandlerBase
 from secop.lib.enum import Enum
 from secop.modules import Attached, Communicator, \
-    Done, Drivable, Module, Readable, Writable
+    Done, Drivable, Module, Readable, Writable, HasAccessibles
 from secop.params import Command, Parameter
 from secop.properties import Property
 from secop.proxy import Proxy, SecNode, proxy_class
@@ -39,3 +39,8 @@ from secop.io import HasIO, StringIO, BytesIO, HasIodev  # TODO: remove HasIodev
 from secop.persistent import PersistentMixin, PersistentParam
 from secop.rwhandler import ReadHandler, WriteHandler, CommonReadHandler, \
     CommonWriteHandler, nopoll
+
+ERROR = Drivable.Status.ERROR
+WARN = Drivable.Status.WARN
+BUSY = Drivable.Status.BUSY
+IDLE = Drivable.Status.IDLE