Merge pull request #650 from daniel-goldstein/exact-topology-distributions

Implement algorithm for calculating species tree topology distributions.

Author: mergify[bot]

docs/python-api.rst

@@ -502,6 +502,8 @@ See :ref:`sec_combinatorics` for details.
 
 .. autofunction:: tskit.all_tree_labellings
 
+.. autoclass:: tskit.TopologyCounter
+
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 Linkage disequilibrium
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