def field(tag, modifier = nil)
      fields.detect { |field| field.tag == tag.to_s && (modifier.nil? || field.modifier == modifier) }
    end
def field(tag, modifier = nil)
      fields.detect { |field| field.tag == tag.to_s && (modifier.nil? || field.modifier == modifier) }
    end
Internal: Field accessor
 returns a field object by a given tag

 Example:
 field(20)
 field(61,'F')