Make transferFlags a multivariate function

It is not uncommon to transfer the flags from multiple fields to one target, e.g tansferFlags(field=["a", "b", "c"], target="x"). Currently we only support this by calling transferFlagsmultiple times, which might trash up the history with empty columns. This MR makestransferFlags` a function supporting the projection of flags from many to one.

saqc/funcs/flagtools.py

@@ -376,7 +376,11 @@ class FlagtoolsMixin:
             if target not in self._data:
                 self._data[target] = pd.Series(np.nan, index=self._data[field].index)
                 self._flags._data[target] = History(self._data[target].index)
-
+            if not self._data[field].index.equals(self._data[target].index):
+                raise ValueError(f'All Field and Target indices must match!\n'
+                                 f'Indices of {field} and {target} seem to be not congruent within the context of the given\n'
+                                 f'- fields: {fields}\n '
+                                 f'- and targets: {targets}')
             history = self._flags.history[field].copy(deep=True)
 
             if overwrite is False: