low point averaged scoring

I've added an option to the series scoring tab to optionally average
a boats non-discarded race scores to create a series score, rather
then the more usual accumulation of the non-discarded race scores.
As mentioned by USSA in of their "Alternative scoring systems for
long series" note.

This will be available in v1.47 beta 2 due for release shortly.

Regards,
Colin J
www.sailwave.com