Feature request: alternate RtRoiCombine::getWeightedAve method

[pwighton]

We would like an additional RtRoiCombine::getWeightedAve() method, perhaps called getAvgWeightedSum() that, like getWeightedAve(), computes the weighted sum, but instead of dividing by the sum of the weights, divides by the number of voxels in the mask.

I believe the intent here is to aggregate z-scores over a region.