def map axis=:vector, &block
      dispatch_to_axis_pl axis, :map, &block
    end
Map over each vector or row of the data frame according to
the argument specified. Will return an Array of the resulting
elements. To map over each row/vector and get a DataFrame,
see #recode.

== Description

The #map iterator works like Array#map. The value returned by
each run of the block is added to an Array and the Array is
returned. This method also accepts an axis argument, like #each.
The default is :vector.

== Arguments

* +axis+ - The axis to map over. Can be :vector (or :column) or :row.
Default to :vector.