Dynamic parameters should support iterator protocol

[jlstevens]

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.

[jbednar]

I think that making param support iterators is as simple as changing the produce_value() function in param/init.py to first check to see if it's iterable ("try x=iter(value_obj) except: ..."), and if so to iterate it and return the result, and if not to check if it's callable, and if so to call it and return the result.

Then it would make sense to change the NumberGenerator class in ImaGen to support the iterator interface. At a minimum the iterator interface could just invoke the call method on the NumberGenerator classes, which would make it easy to mix and match different Param and ImaGen versions. Or NumberGenerator could support only the iterator interface, in which case ImaGen would need to require a specific Param version (which shouldn't be a big problem either).

[ceball]

Is "Use iterators and generators with Dynamic parameters" #32 out of date now?