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self._trafo \elem {1,2,3}. Therefore, either list needs to have
four items or should be accessed via [self._trafo - 1].
This way seem more readable to me.

Bug can be triggered by creating a FFTOperator `op` and calling
`print(op.inverse.adjoint)`.

added outer product operator, added outer method to field and linearisation, added integrate method to linearisation,

Related to Issue #62