Prob combined moves

This adds the option to combine TakeSteps with specified probabilities.
In contrast to TakeStepPattern, the steps are then iterated over according to their specified relative probabilities (frequencies, weights, ...).

This goes back to the discussion about TakeStepPattern we had earlier.
this naming confused me for a while, I thought it would be a pointer to the step class.  I would rename m_current_step to something like m_current_index