fix missing import in change message

Transported values in a change must be converted first.
As this is only relevant for the exotic "scaled" and "blob"
datatypes, this was not detected yet.

- add tests
- suppress warning PytestUnhandledThreadExceptionWarning in tests
+ change import_value methods to raise no other exceptions than
  WrongTypeError and RangeError
+ simplify Command.do: as import_value already raises the
  appropriate error, no more try/except is needed

Change-Id: I299e511468dc0fcecff4c20cf8a917da38b70786
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/32743
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy/protocol/dispatcher.py

@@ -162,7 +162,9 @@ class Dispatcher:
         if pobj.readonly:
             raise ReadOnlyError(f"Parameter {modulename}:{pname} can not be changed remotely")
 
-        # validate!
+        # convert transported value to internal value
+        value = pobj.datatype.import_value(value)
+        # verify range
         value = pobj.datatype.validate(value, previous=pobj.value)
         # note: exceptions are handled in handle_request, not here!
         getattr(moduleobj, 'write_' + pname)(value)