It's a PITA to create a new local Distribution

[kwmsmith]

We should make it easier to make a new local Distribution -- perhaps allow creating a new distribution from an existing distribution.

This comes up when a new local arrays are created that don't match the distribution of the originating array -- perhaps the local shape changes, or the local ndims. local.maps.Distribution could have a classmethod like from_distribution that would take an existing distribution for the defaults and allow passing in new_shape, new_dist and new_grid_shape args that would set the appropriate values. There may need to be global communication to support this.