Dynamic parameters should support iterator protocol

Dynamic parameters currently accept callable objects which are called to generate new values (e.g. with numbergen). It would make sense (and probably be more Pythonic!) to use the iterator protocol instead, calling the next function (for Python 3 compatibility) to get each value in turn.

It should be relatively easy to update numbergen to support the iterator protocol - it may be possible to deprecate the callable interface without too much trouble.