Use public properties domain & codomain in `LinearOperator.__neg_…

…_` (#410)

The `__neg__` function in class `LinearOperator` uses the `_domain` and
`_codomain` attributes, which is wrong since these may not be defined.
It should use the exposed (interface) properties `domain` and `codomain`
instead.

psydac/linalg/basic.py

@@ -279,7 +279,7 @@ def __neg__(self):
         a new object of the class ScaledLinearOperator.
         
         """
-        return ScaledLinearOperator(self._domain, self._codomain, -1.0, self)
+        return ScaledLinearOperator(self.domain, self.codomain, -1.0, self)
 
     def __mul__(self, c):
         """