Extend HTN generation to handle nested parallel/sequence/choose statements

[dcerys]

For generating HTNs from Pamela, there is a current restriction that parallel/sequence/choose statements cannot be nested within another. For example, the following isn't allowed:

(defpmethod main []
  (sequence
   (parallel
    (one)
    (two))
   (three)))

The workaround is trivial; just refactor the nested statement into a separate method. For example:

(defpmethod main []
  (sequence
   (one-and-two)
   (three)))

(defpmethod one-and-two []
  (parallel
    (one)
    (two)))

It would be a low-priority enhancement to support methods containing nested forms, as in the original main above.