implement SECoP proxy modules

A proxy module is a module with a known structure, but
accessed over a SECoP connection.
For the configuration, a Frappy module class has to be given.
The proxy class is created from this, but does not inherit from it.
However, the class of the returned object will be subclass of the
SECoP base classes (Readable, Drivable etc.).
A possible extension might be, that instead of the Frappy class,
the JSON module description can be given, as a separate file
or directly in the config file.
Or we might offer a tool to convert the JSON description to
a python class.

Change-Id: I9212d9f3fe82ec56dfc08611d0e1efc0b0112271
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22386
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/__init__.py

@@ -25,8 +25,10 @@
 
 # allow to import the most important classes from 'secop'
 from secop.datatypes import *
+from secop.lib.enum import Enum
 from secop.modules import Module, Readable, Writable, Drivable, Communicator, Attached
 from secop.params import Parameter, Command, Override
 from secop.metaclass import Done
 from secop.iohandler import IOHandler, IOHandlerBase
 from secop.stringio import StringIO, HasIodev
+from secop.proxy import SecNode, Proxy, proxy_class