forbidden new columns are thrown away silently

brief

if a (user)-registered function sets a flags-column, which is not specified in the decorator, the column is silently thrown away

example

@flagging()
def flagFoo(data, field, flags, **kwargs):
   flags["spam"] = flags[field]
   return data, flags

If the flags columns are a,b and one calls

qc.flagFoo('a')

it just works fine and the result still only have a,b.

expected behavior

SaQC._validate() should be triggered and raise and Error