Replace InverseOperator and AdjointOperator with OperatorAdapter, and more

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.