Performance check (in ift.check_operator) broken for BernoulliEnergy

The operator performance check introduced in a50ec021 changes the operator test position by a factor of two for the 'application to a linearization' test. This breaks the test for the BernoulliEnergy, as it creates input values outside the interval of [0, 1).

@reimar, since you coded this function: does the factor of two serve a valuable purpose, or can we safely delete it?