Increase test tolerance a little

The issue is that IO involves serialisation to string and potential unit conversion, which is inherently brittle. We just care to catch accidental unit errors or accidental forgotten parameters etc, so raising the relative tolerance should be fine.

test/atomsbase.jl

@@ -86,5 +86,5 @@ end
         ExtXYZ.save(outfile, system)
         ExtXYZ.load(outfile)::AbstractSystem
     end
-    test_approx_eq(system, io_system; rtol=1e-6)
+    test_approx_eq(system, io_system; rtol=1e-4)
 end