Define the getWeight method in ReachGen.
This is in preparation for forthcoming changes to WeightGen.
WeightGen::getWeight: make pure virtual.

SemElem::one is not necessarily a sound default value for getWeight to return. Also, this forces WeightGen writers to think about the behavior they want for getWeight.
WeightGen::getWildWeight: assert false

getOne is not necessarily a sound default value for getWildWeight (see previous commit). But since most analysis writers do not deal with wild weights, the method is not made pure virtual so they don't have to override it.
NWA query::doForwardAnalysis and poststar: pass the initial weight as a parameter

Note: you can always pass in WeightGen::getOne() as the value for the initial weight.