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.