Fix failing empty array tests.
Test some of the attributes of the empty slice.
Check attributes on empty 2d slice.
Pass the Distribution object into from_localarrays.

This gets around the fact that `n` maps are translated into `b` maps on the Local side.
Add some tests of the output distribution.
Merge pull request #459 from enthought/bugfix/446

Fix empty slice regression
Merge pull request #460 from enthought/bugfix/443

Fix #443
Fixes #461.

Generalizes `Distribution.is_compatible()` to treat 'n' as compatible
with 'b', 'c', and 'n' when it makes sense to do so.