persistent params / trinamic motor

for the trinamic motor we need persistent parameters

Change-Id: Id509b87f8368ea5ba1aca71951f79433b0b4b79f
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/26405
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/core.py

@@ -37,3 +37,4 @@ from secop.poller import AUTO, DYNAMIC, REGULAR, SLOW
 from secop.properties import Property
 from secop.proxy import Proxy, SecNode, proxy_class
 from secop.io import HasIodev, StringIO, BytesIO
+from secop.persistent import PersistentMixin, PersistentParam