Issue15 fix version 2

Better bug fix for integer overflow of #15, should be used instead of #17. No changes to internal Curve representation, only cast to double before performing totalPositives * totalNegatives operation in rocArea and mannWhitneyU.

I'm casting to double instead of long since the ultimate result of the expression is always a double. If you think a more accurate computation will occur by casting to long, performing the multiplication, and only then casting to double I can do it but that involves more casts and seems confusing.