Use number of particles for tally normalization #1442
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pshriwise
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As described in #1417, our tallies are currently normalized based on the total starting weight of particles in a batch, which can potentially cause problems with variance reduction schemes. In principle, if you halve the weight of the starting particles, you should get tally results that are half as much. This PR changes that normalization. Note that the normalization for k-effective tallies still uses the total weight to get the correct value.
Some of the CMFD test results changed ever so slightly because of this new normalization (in CMFD, we are changing the weights of source particles, so roundoff differences do ensue). I also added a tally in our one test that uses multiple generations per batch to ensure that we are not messing things up there.
A review from @shimwell and/or @makeclean would be appreciated. In particular, does this fix any issues you guys were running into?
Closes #1417.