[Changelog] mention new entity networks and gmsh writer

CHANGELOG.md

@@ -2,6 +2,10 @@ Differences between Frackit 1.2 and Frackit 1.1
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 ### Improvements and Enhancements
+- The `GmshWriter` class now allows setting the mesh sizes for individual entities, subdomains, intersections or
+  intersection junctions of the network.
+- The `EntityNetwork` class has been revised and now allows for iteration over all fragments of the network.
+  Moreover, mappings are provided to retrieve back the primary identifier of the entities to which the fragments belong.
 - An implementation of `Sphere` has been added to the classes of available geometries
 - Three-dimensional `Polygon` class has been implemented
 - A sampler class for 3d polygons, `PolygonSampler`, has been added