add "bowNP_sip" to `fc` using `tfidf` data

def add_sip_to_fc(fc, tfidf, limit=40):
    '''add "bowNP_sip" to `fc` using `tfidf` data
    '''
    if 'bowNP' not in fc:
        return
    if tfidf is None:
        return
    sips = features.sip_noun_phrases(tfidf, fc['bowNP'].keys(), limit=limit)
    fc[u'bowNP_sip'] = StringCounter(sips)