Add systematic support for SumOfWeightAlg
The SumOfWeightsAlg introduced with !494 (merged) should be updated to include proper systematics support for all systematics affecting the weights. This should follow an approach similar to !538 (merged)
The SumOfWeightsAlg introduced with !494 (merged) should be updated to include proper systematics support for all systematics affecting the weights. This should follow an approach similar to !538 (merged)