def to_nmatrix axis=:horizontal
      unless numeric? && !include?(nil)
        raise ArgumentError, 'Can not convert to nmatrix'\
          'because the vector is numeric'
      end

      case axis
      when :horizontal
        NMatrix.new [1, size], to_a
      when :vertical
        NMatrix.new [size, 1], to_a
      else
        raise ArgumentError, 'Invalid axis specified. '\
          'Valid axis are :horizontal and :vertical'
      end
    end
Convert vector to nmatrix object
@param [Symbol] axis :horizontal or :vertical
@return [NMatrix] NMatrix object containing all values of the vector
@example
dv = Daru::Vector.new [1, 2, 3]
dv.to_nmatrix
# =>
# [
#   [1, 2, 3] ]