[pyclustering.cluster.bsas][ccore.clst.bsas] Clustering of stream data

**Introduction**
Sequential algorithm BSAS can be used for stream processing. Current implementation requires whole dataset at once and adding new point requires to repeat processing again. Therefore new method is required to continue processing is required.

**Description**
Method 'continue(self, data)' should be implemented. Where 'data' is containers of added points. When 'continue()' finish processing, methods 'get_clusters()' and 'get_representatives()' should returned new clusters and representatives.

Interface between pyclustering and its core should be also updated.