Allow 6 point rule in ptd_calculate_point_type()

[jsgm21]

Currently, ptd_calculate_point_type() uses a 7 point rule (both for trend and consistently above/below mean). A points_for_significance parameter could be useful in ptd_spc() to allow users to choose between 6 or 7 points, as allowed in MDC's Excel tool.

Thanks for looking into this.