Protect against multiple offsets

This pull request is a modification to the mixing module to guarantee that the offsets are applied to the hits only once.  The offsets are applied up front by the MixingModule using a new "Adjuster" class.  With this fix, neither the digit accumulators nor the CrossingFrames can apply the shifts, as they do not have write access to the hits.

This fix has been regression tested, but it needs to be tested that it does indeed fix the problem, and does not introduce other problems. In particular, it should be tested that the shifts are in fact applied.