feature(dataloader): get sub_df to return a smaller data frame

dataloader.py

@@ -509,6 +509,30 @@ class dataloader:
         """
         return self.df.pivot(y, x, signal)
 
+    def sub_df(self, signal, duration_threshold):
+        """
+        Find a sub dataframe of dataloader's main dataframe.
+
+        Parameters
+        ----------
+        duration_threshold: float
+            Specifies the fraction of the total duration of the time-lapse
+            experiment for which a cell must be present in a trap.
+        """
+        if duration_threshold < 0 or duration_threshold > 1:
+            print(
+                f"The threshold must be a fraction, not {duration_threshold}."
+            )
+            return
+        else:
+            wdf = self.wide_df(signal)
+            keep = (
+                wdf.notna().sum(axis=1) > duration_threshold * wdf.columns.size
+            ).values
+            ids_to_keep = list(wdf.index[keep])
+            sdf = self.df[self.df.id.isin(ids_to_keep)]
+            return sdf
+
     def get_time_series(self, signal, group=None):
         """
         Extract a signal as a 2D array with each row a time series.