Fix for issue #225 (#226)
Fix for issue #225 (#226)

* Fix 'num_iterations' attribute of Run.

* Refactor Walker.children for performance.

* Revise docstrings.

* Handle initial states that are not generated from a basis state.

* Fix bug in Walker.children.

* Fix bug in Walker.children.

* Revise docstrings for consistency with WESTPA 2.0 paper.
westpa/westpa