autogeneration of iodevs

allow a shortcut for generating iodevs:
If 'uri' is given in the config file for a module with HasIodev, it
will autogenerate its communicator.
The iodevClass attribute on the module class determines the class
for the iodev to be generated.

Change-Id: I4e82a57a33218fd159cec9f4c95171365c55d94a
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22068
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>

cfg/ls370test.cfg

@@ -6,21 +6,16 @@ type = tcp
 bindto = 0.0.0.0
 bindport = 5000
 
+[module lsmain]
+class = secop_psi.ls370res.Main
+description = main control of Lsc controller
+uri = localhost:4567
+
 [module res]
 class = secop_psi.ls370res.ResChannel
 vexc = '2mV'
-.channel = 3
-.description = resistivity
-.main = lsmain
-.iodev = lscom
-
-[module lsmain]
-class = secop_psi.ls370res.Main
-.description = main control of Lsc controller
-.iodev = lscom
-
-[module lscom]
-class = secop_psi.ls370res.StringIO
-.uri=localhost:4567
-.description = serial communicator to an LS 370
-.visibility = 3
+channel = 3
+description = resistivity
+main = lsmain
+# the auto created iodev from lsmain:
+iodev = lsmain_iodev