drivers for AH700 and K2601B

AH2700: Andeen Hagerling capacitance bridge

This is a use case for 'return Done' in read_<parameter>

K2601b: Keithley source meter

Current and Source are two dependend SECoP modules: only
one of them might be active. Writing to target is allowed only
when active, but both values are always readable.
A common SourceMeter bare Module is present for common functionality.

Change-Id: I6f1875298ef928bcc2d60b89560b837139160775
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22071
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

cfg/ah2700test.cfg

@@ -0,0 +1,17 @@
+[node AH2700Test.psi.ch]
+description = AH2700 capacitance bridge test
+
+[interface tcp]
+type = tcp
+bindto = 0.0.0.0
+bindport = 5000
+
+[module cap]
+class = secop_psi.ah2700.Capacitance
+description = capacitance
+uri=ldmse3-ts:3015
+
+#[module ahcom]
+#class = secop_psi.ah2700.StringIO
+#uri=ldmse3-ts:3015
+#description = serial communicator to an AH2700