def each axis=:vector, &block
      dispatch_to_axis axis, :each, &block
    end
Iterate over each row or vector of the DataFrame. Specify axis
by passing :vector or :row as the argument. Default to :vector.

== Description

`#each` works exactly like Array#each. The default mode for `each`
is to iterate over the columns of the DataFrame. To iterate over
rows you must pass the axis, i.e `:row` as an argument.

== Arguments

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