refine_post_merge_boundaries() could remove double-counted pixels

[jni]

refine_post_merge_boundaries in agglo.py was created for the purpose of maintaining the integrity of 0-labeled boundaries in cases such as the following:

1 0 2 0
1 0 0 3
0 4 0 3

When we have a merge of body 3 into body 2, a new pixel is added to the boundary between 1 and 2. If this addition is not noted, when 1 and 4 are merged, that pixel will be removed and the boundary between 1 and 2 will have a leak.

However, it turns out that this function could be used to detect when boundary pixels are double-counted during a merge and correct the double counting.

    idxs = set(boundaries_to_edit[(u,v)])                            
    if self.has_edge(u, v):                                          
        idxs = idxs - self[u][v]['boundary']                         
        self[u][v]['boundary'].update(idxs)                          
        self.feature_manager.pixelwise_update_edge_cache(self, u, v, 
                            self[u][v]['feature-cache'], list(idxs)) 

Find the intersection of idxs and self[u][v]['boundary'] and use the feature_manager to pixelwise_update with remove=True.