make secop.poller.Poller default

modules using the old poller (now called secop.poller.BasicPoller)
need to explcitly declare it (pollerClass = BasicPoller)

BasicPoller is still used in:

secop_mlz/amagnet.py
secop_mlz/entangle.py
secop/simulation.py

Remark: before removing BasicPoller we may need a replacement for
Readable.pollParams

Change-Id: If1ae8b68e02f13e601334656b818337c882e06cc
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21910
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/server.py

@@ -185,10 +185,7 @@ class Server:
         for modname, modobj in self.modules.items():
             self.log.info('registering module %r' % modname)
             self.dispatcher.register_module(modobj, modname, modobj.properties['export'])
-            try:
-                modobj.pollerClass.add_to_table(pollTable, modobj)
-            except AttributeError:
-                pass
+            modobj.pollerClass.add_to_table(pollTable, modobj)
             # also call earlyInit on the modules
             modobj.earlyInit()