Move new method below constructors.
Clean up Distribution slice tests.
Slightly expand a Distribution.slice test.
Add a Distribution.slice test.
Generalize sanitize_indices for incomplete indexing.

and test.
Fill out sanitize_indices docstring.
Remove a competing sanitize_indices.
Add more tests to test_distarray.  Some fail.
Add a call to positivify.
Call positivify in sanitize_indices...

and use sanitize_indices everywhere.
Whitespace.
Call `sanitize_indices` with full args.
Rename a value more descriptively.