Improved delta values to support non-numerical values

Delta values now work for non-numerical values. Any column,
when its value differs from its ref, becomes orange.

A column that was already a "delta column" keeps its previous
behavior (dupe cells for these columns are always displayed in
orange).

Sorting behavior, when Dupes Only and Delta Values are enabled
at the same time, has also been extended to non-numerical
values, making it easy to mass-mark dupe rows with orange
values.

Documentation was updated, unit tests were added.

Fixes #213

core/gui/result_table.py

@@ -21,6 +21,30 @@ class DupeRow(Row):
         self._dupe = dupe
         self._data = None
         self._data_delta = None
+        self._delta_columns = None
+    
+    def is_cell_delta(self, column_name):
+        """Returns whether a cell is in delta mode (orange color).
+        
+        If the result table is in delta mode, returns True if the column is one of the "delta
+        columns", that is, one of the columns that display a a differential value rather than an
+        absolute value.
+        
+        If not, returns True if the dupe's value is different from its ref value.
+        """
+        if not self.table.delta_values:
+            return False
+        if self.isref:
+            return False
+        if self._delta_columns is None:
+            # table.DELTA_COLUMNS are always "delta"
+            self._delta_columns = self.table.DELTA_COLUMNS.copy()
+            dupe_info = self.data
+            ref_info = self._group.ref.get_display_info(group=self._group, delta=False)
+            for key, value in dupe_info.items():
+                if ref_info[key] != value:
+                    self._delta_columns.add(key)
+        return column_name in self._delta_columns
     
     @property
     def data(self):