Add Exchange referencing test

Author: rpep

tests/test_regression_referencing.py

@@ -0,0 +1,32 @@
+# The idea behind these tests is to prevent regression back to a bug which was
+# caused by incorrect referencing. This bug caused
+
+import fidimag
+
+def setup_fixture_micro():
+    mesh = fidimag.common.CuboidMesh(nx=10, ny=10, nz=10,
+                                     dx=1, dy=1, dz=1,
+                                     unit_length=1e-9)
+    sim = fidimag.micro.Sim(mesh)
+    return mesh, sim
+
+
+def setup_fixture_atomistic():
+    mesh = fidimag.common.CuboidMesh(nx=10, ny=10, nz=10,
+                                     dx=1, dy=1, dz=1,
+                                     unit_length=1e-9)
+    sim = fidimag.atomistic.Sim(mesh)
+    return mesh, sim
+
+
+def test_exchange_Ms_regression_micro():
+    mesh, sim = setup_fixture_micro()
+    A1 = 2e-11
+    Ms1 = 8.5e5
+    Ms2 = 9.0e5
+    sim.set_m(Ms1)
+    exch = fidimag.micro.UniformExchange(A1)
+    sim.add(exch)
+    sim.set_m(Ms2)
+    assert exch.Ms is sim._magnetisation, "The Ms in the Exchange Micro class is not a reference to Ms in the Sim class"
+    assert exch.Ms_inv is sim._magnetisation_inv, "The Ms_inv in the Exchange Micro class is not a reference to Ms_inv in the Sim Class"