feature(dataloader.subdf): added group

src/wela/dataloader.py

@@ -598,6 +598,7 @@ class dataloader:
         duration_threshold=None,
         tmin=None,
         tmax=None,
+        group=None,
     ):
         """
         Find a sub data frame of dataloader's main data frame.
@@ -615,8 +616,13 @@ class dataloader:
             Only include data for times greater than tmin
         tmax: float (optional)
             Only include data for times less than tmax
+        group: str (optional)
+            Group to specialise to.
         """
-        sdf = self.df
+        if group is None:
+            sdf = self.df
+        else:
+            sdf = self.df[self.df.group == group]
         selected_ids = []
         # drop signals that are all NaN
         if dropna and signal: