def vector_by_calculation &block
      a = each_row.map { |r| r.instance_eval(&block) }

      Daru::Vector.new a, index: @index
    end
DSL for yielding each row and returning a Daru::Vector based on the
value each run of the block returns.

== Usage

a1 = Daru::Vector.new([1, 2, 3, 4, 5, 6, 7])
a2 = Daru::Vector.new([10, 20, 30, 40, 50, 60, 70])
a3 = Daru::Vector.new([100, 200, 300, 400, 500, 600, 700])
ds = Daru::DataFrame.new({ :a => a1, :b => a2, :c => a3 })
total = ds.vector_by_calculation { a + b + c }
# <Daru::Vector:82314050 @name = nil @size = 7 >
#   nil
# 0 111
# 1 222
# 2 333
# 3 444
# 4 555
# 5 666
# 6 777