introduced Datatype.copy

as Datatype got mutable, it has to be copied when inherited.
Params.copy must call the introduced method Datatype.copy.

in addition:
- fixed bugs in ScaledInteger.__repr__ and datatypes.DATATYPES['struct']
- do not export unit from Parameters

Change-Id: Id552c33843b1b2bedffc68d1bd909705dcfb5605
Reviewed-on: https://forge.frm2.tum.de/review/20324
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_params.py

@@ -46,6 +46,10 @@ def test_Parameter():
     assert p1.ctr != p2.ctr
     with pytest.raises(ValueError):
         Parameter(None, datatype=float)
+    p3 = p1.copy()
+    assert repr(p1) == repr(p3)
+    assert p1.datatype != p3.datatype
+
 
 def test_Override():
     p = Parameter('description1', datatype=BoolType, default=False)