Dev/guess clusters (#355)

- exposing ClusterVector::empty in python
- added member guess_n_clusters

python/tests/test_ClusterFile.py

@@ -14,6 +14,8 @@ from conftest import test_data_path
 def test_cluster_file(test_data_path): 
     """Test ClusterFile""" 
     f =  ClusterFile(test_data_path / "clust/single_frame_97_clustrers.clust") 
+    assert f.estimate_n_clusters() == 97
+    assert f.tell() == 0
     cv = f.read_clusters(10) #conversion does not work
 
 
@@ -62,4 +64,4 @@ def test_read_clusters_and_fill_histogram(test_data_path):
         hist_py = pickle.load(f)
         
     #Compare the two histograms
-    assert hist_aare == hist_py
+    assert hist_aare == hist_py