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

@@ -19,7 +19,7 @@ from saqc import BAD, FILTER_ALL, UNFLAGGED
 from saqc.core import DictOfSeries, flagging, register
 from saqc.core.history import History
 from saqc.lib.checking import validateChoice, validateWindow
-from saqc.lib.tools import initializeTargets, isflagged, isunflagged, toSequence
+from saqc.lib.tools import initializeTargets, isflagged, isunflagged, multivariateParameters, toSequence
 
 if TYPE_CHECKING:
     from saqc import SaQC
@@ -290,6 +290,7 @@ class FlagtoolsMixin:
         demask=[],
         squeeze=[],
         handles_target=True,  # function defines a target parameter, so it needs to handle it
+        multivariate=True,
     )
     def transferFlags(
         self: "SaQC",
@@ -349,37 +350,39 @@ class FlagtoolsMixin:
            0   -inf   -inf   -inf
            1  255.0  255.0  255.0
         """
-        history = self._flags.history[field]
-
-        if target is None:
-            target = field
-
-        if overwrite is False:
-            mask = isflagged(self._flags[target], thresh=kwargs["dfilter"])
-            history._hist[mask] = np.nan
-
-        # append a dummy column
-        meta = {
-            "func": f"transferFlags",
-            "args": (),
-            "kwargs": {
-                "field": field,
-                "target": target,
-                "squeeze": squeeze,
-                "overwrite": overwrite,
-                **kwargs,
-            },
-        }
-
-        if squeeze:
-            flags = history.squeeze(raw=True)
-            # init an empty history to which we later append the squeezed flags
-            history = History(index=history.index)
-        else:
-            flags = pd.Series(np.nan, index=history.index, dtype=float)
 
-        history.append(flags, meta)
-        self._flags.history[target].append(history)
+        fields, targets = multivariateParameters(field, target)
+
+        for field, target in zip(fields, targets):
+
+            history = self._flags.history[field]
+            # append a dummy column
+            meta = {
+                "func": f"transferFlags",
+                "args": (),
+                "kwargs": {
+                    "field": field,
+                    "target": target,
+                    "squeeze": squeeze,
+                    "overwrite": overwrite,
+                    **kwargs,
+                },
+            }
+
+            if overwrite is False:
+                mask = isflagged(self._flags[target], thresh=kwargs["dfilter"])
+                history._hist[mask] = np.nan
+
+            if squeeze:
+                flags = history.squeeze(raw=True)
+                # init an empty history to which we later append the squeezed flags
+                history = History(index=history.index)
+            else:
+                flags = pd.Series(np.nan, index=history.index, dtype=float)
+
+                history.append(flags, meta)
+                self._flags.history[target].append(history)
+
 
         return self