Add swap particle move takestep

This adds a takestep class doing particle pair swapping.
To accommodate that, mc is changed such that it loops over a vector of steps (moves) similar to the looping over actions and configuration tests.

I think the signature of these should be 

    void increase_acceptance(double factor)

where factor would be how much to change the stepsize by.  It wouldn't make sense for integer stepsizes, (e.g. number of particle swaps), but in those cases it can simply be ignored.