Define the getWeight method in ReachGen.
This is in preparation for forthcoming changes to WeightGen.
Make WeightGen::getWeight 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.
By default, getWildWeight performs an assert(false). getOne is not necessarily a sound default value for getWildWeight. But since most analysis writers do not deal with wild weights, the method is not made pure virtual.
doForwardAnalysis and poststar take the initial weight to be used as a parameter.
Note: you can always pass in WeightGen::getOne() as the value for the initial weight.