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_mlz/amagnet.py

@@ -31,7 +31,7 @@ import math
 from secop.datatypes import ArrayOf, FloatRange, StringType, StructOf, TupleOf
 from secop.errors import ConfigError, DisabledError
 from secop.lib.sequence import SequencerMixin, Step
-from secop.modules import Drivable, Parameter
+from secop.modules import Drivable, Parameter, BasicPoller
 
 
 class GarfieldMagnet(SequencerMixin, Drivable):
@@ -47,6 +47,8 @@ class GarfieldMagnet(SequencerMixin, Drivable):
     the symmetry setting selects which.
     """
 
+    pollerClass = BasicPoller
+
     parameters = {
         'subdev_currentsource': Parameter('(bipolar) Powersupply', datatype=StringType(), readonly=True, export=False),
         'subdev_enable': Parameter('Switch to set for on/off', datatype=StringType(), readonly=True, export=False),