Fix addTerm(String, double) to respect stoplist

[bodom0015]

• FeatureVector#addTerm(String term, double weight) should now respect the stoplist

[gtsherman]

We discussed earlier that this PR is a good idea. Other things to think about with FeatureVector for discussion with @craig-willis:

• setTerm is rarely used in client code -- maybe change to protected?
• More importantly, setTerm has a bug in that the feature length is updated by adding the new weight without first subtracting its old weight
• The clarity method is perhaps better housed somewhere else
• It can be confusing which feature receives the weight in the interpolate method, though I have no immediate suggestions for fixing
• Also within the interpolate method, is it really the right thing to do to ignore mixing weights outside of [0, 1]?
• The main method should probably be killed