Adds distribution() convenience function.

[kwmsmith]

I'm tired of typing Distribution.from_shape all the time. This seems
to be the main entry point, so distribution is a shortcut.

[coveralls]

Coverage decreased (-0.02%) when pulling 6aac2b5 on feature/distribution-creation into 7a7bf40 on master.

[bgrant]

One down side- I often name my instances distribution :)

[kwmsmith]

OK, what about this design then:

• Distribution.from_shape ==> Distribution.__init__
• New classmethod Distribution.from_maps
• Distribution.__init__ and all other alternative constructors call Distribution.from_maps to make a Distribution object.

I'd really like a shorter name for what is now Distribution.from_shape, since that is by far the most common and convenient way to make distribution objects.

[bgrant]

Haha, yeah, that's probably the way to go. I think that's the way it was before, and we changed it to this way because we wanted the most general constructor to be __init__.

[bgrant]

We have considered two design principles for alternate constructors:

1. __init__ is the most general method, and all other constructors call it
2. __init__ is the most commonly used method, and it likely calls another, more general, classmethod constructor (as do the other alternate constructors).

I'm leaning towards (2.) now, as makes sense for the shortest name to call the most commonly-used method. I seem to remember Raymond Hettinger recommending that as well. I do think we should try, as much as possible, to have all constructors call an underlying general constructor; else we can end up with bugs where some constructors don't create all the same instance attributes as others.

[bgrant]

@kwmsmith: Should we close this PR and make an issue?

[kwmsmith]

Closing. Will be addressed by a different PR.