sampler: support custom iterator for on_sample()
Currently, only a fixed interval (every) is supported by the function on_sample(), in the case of non-uniform grids (e.g. as in blocking_scheme), this is not enough.
Ideally, one would provide an iterator or a list of time steps to on_sample()
For a discussion, see http://article.gmane.org/gmane.science.simulation.halmd.devel/438