Allow use of a single scatter window

[pearsomark]

Modify get_scatter_from_TEW to work with a lower scatter window only.

[pearsomark]

I don't have permission to create a new branch.

[lukepolson]

@pearsomark I need to update the developers guide; what you should do if you want to contribute is create a new fork, clone your fork locally, make the changes, push to your fork on github, and then submit a pull request.