Python syntax specification of caesar rules

[hughdickinson]

Adds the ability to specify workflow rules using a Python format. For example the following Python-like expression:

"(subject['#hidden_val'] == 0) and [some_reducer['counts'], 0] < 10"

will be translated into:

['and',
 ['eq', ['lookup', 'subject.#hidden_val'], ['const', 0]],
 ['lt', ['lookup', 'some_reducer.counts', ['const', 0]], ['const', 10]]]

Expressions are legal Python except for the list-like syntax used to express lookups with default values.

Reducer names are checked against the list of reducers specified for the workflow, but the reducer attributes are not validated. Possible extensions would be specifying a shema for each type of reducer.

[hughdickinson]

Just wondering whether there is a blocking issue with this PR that needs me to fix it or it's just been on the back burner.

[hughdickinson]

Same query as the one I made in January. If changes are needed, let me know.