Copy global_dim_data docstring to Distribution

Its __init__ method takes a global_dim_data, and from_global_dim_data will go away soon.
Fix Distribution.__init__ to pass ddpr...

instead of the less general shape/dist/grid_shape.
Remove Context.from_global_dim_data...

and fix tests to first make a Distribution object from a global_dim_data, then make a DistArray with the Distribution object.
Merge pull request #351 from enthought/refactor/remove-from-global-dim-data

Remove `Context.from_global_dim_data`