Finds the semantic similarity (inverse of the semantic distance)
        between two GO terms.

def semantic_similarity(go_id1, go_id2, godag, branch_dist=None):
    '''
        Finds the semantic similarity (inverse of the semantic distance)
        between two GO terms.
    '''
    dist = semantic_distance(go_id1, go_id2, godag, branch_dist)
    if dist is not None:
        return 1.0 / float(dist)