Check for empty localarrays when Distributions are made

[cowlicks]

This adds checks to all the Distribution creation paths to make sure that no empty localarrays are created. This should fail several tests until PR #444 is merged. This closes issue #445.

This also adds a Distribution.localsizes instance method, which returns the sizes of the localarrays.

[cowlicks]

Note that this has to duplicate the check for empty localarrays three times. Two for each Distribution classmethod, and one for Distribution's __init__. It would be nice if the Distribution classmethods were refactored to call Distribution's __init__ so we could avoid this duplication.

[kwmsmith]

@cowlicks please rebase against master.

[cowlicks]

It looks like we have some conflicting implementations here. This PR checks that we don't create any empty LocalArrays, but it looks like that is intended behavior in some of the slicing PR's. See this

[bgrant]

Postponing to 0.7.