Change GraphTopologyConversion::convert return value to include the new GraphIsomorphism

This allows to convert maps for the converted graph.

src/maps.rs

@@ -94,6 +94,35 @@ impl<E: Copy, T: Clone> ElementMap<E, T> {
             to_index: self.to_index,
         }
     }
+
+    /// Creates a new `ElementMap` indexed by elements of type `F`, such that for each element *e*
+    /// of this map that `mapping` maps to some *f*, the new map contains at *f* a clone of the
+    /// value this map contains at *e*.
+    ///
+    /// A slot of `mapping` counts as unmapped if it holds the default value of `mapping`, which
+    /// must therefore never be a valid element of `F`. Unmapped slots are skipped. Slots of the new
+    /// map that no element is mapped to, and slots of this map beyond the length of `mapping` or
+    /// vice versa, are left at this map's default value. The `template` is only used to determine
+    /// the length and the index conversion function of the new map.
+    ///
+    /// Iterates the raw slots of both maps, so it neither needs the graph nor the element handles.
+    pub(crate) fn remap<F: Copy + Eq>(
+        &self,
+        mapping: &ElementMap<E, F>,
+        template: &ElementMap<F, ()>,
+    ) -> ElementMap<F, T> {
+        let mut result = ElementMap {
+            data: vec![self.default.clone(); template.data.len()],
+            default: self.default.clone(),
+            to_index: template.to_index,
+        };
+        for (value, &f) in self.data.iter().zip(&mapping.data) {
+            if f != mapping.default {
+                result.data[(template.to_index)(f)] = value.clone();
+            }
+        }
+        result
+    }
 }
 
 impl<E: Copy, T: Clone> Index<E> for ElementMap<E, T> {