This patch introduces the OperatorAdapter class, which does the work so far done by InverseOperator and AdjointOperator, but it is smaller and more flexible.
It also removes the method inverse_draw_sample() and adds a fromInverse flag to draw_sample() instead, which also makes many things easier to write.
As a completely unrelated bonus, there is an interface to SciPy's conjugate gradient solver, which can now be used as an alternative to Nifty's ConjugateGradient.