Add generator executor

[rabernat]

This PR adds a .to_generator() method to the BaseRecipe class. It allows one to iterate through a recipe as follows:

rec = MyRecipe()
for function, args, kwargs in rec.to_generator():
    function(*args, **kwargs)

This will eventually replace the explicit mode of manual recipe execution and open the door to more flexible recipe patterns (e.g. #224).

[alxmrs]

Nice change! That's excellent that we can iterate through recipes one step at a time.

[rabernat]

I think I am going to merge this as it does not yet remove or deprecate any existing functionality, and it contains significant improvements to the testing and code organization.

In a follow-up PR, I will document this new feature and consider how to deprecate the old manual execution steps.

[cisaacstern]

👍 I am going to use this functionality to run pangeo-forge/staged-recipes#93 today and take notes for the follow-on documentation PR.