add optional validation to ValueType

* add optional parameter for ValueType: validator
used for checking, if a value meets a criteria (e.g. is dict)
+ InternalParameter, which is not exported and can hold any python value

Change-Id: If39a7a4a8019f2aa1a930e42cbef4fca59163b78
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30787
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

frappy/params.py

@@ -163,7 +163,7 @@ class Parameter(Accessible):
         export=False, default=False)
     update_unchanged = Property(
         '''[internal] updates of unchanged values
-        
+
         - one of the values 'always', 'never', 'default'
           or the minimum time between updates of equal values [sec]''',
         OrType(FloatRange(0), EnumType(always=0, never=999999999, default=-1)),