:carry_forward option

[s12chung]

we have this documentation internally in our README.

Empty Cells Implying Previous Cell

Suppose you have a table like thus:

id	name
1	a
*	b

where * is an empty cell, but it really implies “1”, the previous id.

You can memoize and point to the previous cell:

class ProjectImportRow
  def id
    # `#previous` is from `csv_row_model`
    # memoize because `previous.previous` is always `nil`, otherwise `csv_row_model` creates a linked list
    @id ||= original_attribute(:id) || previous.try(:id)
  end
end

want to implement this with a :carry_forward, :or_previous, :take_previous option---I'm open to other words.