Incorrect StdDiff calculation for continuous values

[chrisknoll]

Expected behavior

Distribution StdDiff calculation should follow a published standard.

Actual behavior

The current DistributionItem std diff calculation is slightly different than what is in literature.

Steps to reproduce behavior

See this line of code, and note the proper calculation should use the avg of the sum of squares.

current:

double sd = Math.sqrt(sd1 * sd1 + sd2 * sd2);

proposed:

double sd = Math.sqrt((sd1 * sd1 + sd2 * sd2) / 2.0 );