Use a named tuple class for (weight,immediate); no functional change

Make the WeightChange class, which is a collections.namedtuple. This lets us access the weight and immediate members by name, making the code clearer.

It also reduces some confusion in the previous code where (immediate[,weight]) was used for overrides and (weight,immediate) was used for desired weight changes (i.e. after applying those overrides).

This change has been tested against the test suite on both python2 and python3.