Erase the entry for state in stateInfos when a state is removed
NWA: Added convenience functions for analysis on NWAs:

- Nwa::poststar() creates a query automata and performs poststar on it
- Nwa::readPoststarResult() takes a "poststarred" automaton, computes weights and returns the map from states to the corresponding weights
- Nwa::doForwardAnalysis() returns the analysis result in the form of state-weight map

Signed-off-by: Evan Driscoll <driscoll@cs.wisc.edu>
NWA: Improve comments and some whitespace