tree sequence compatibility

[GertjanBisschop]

• Extract tree-based mutation configurations (phased/unphased, rooted/unrooted). Note that ts.edge_diffs() can be used to do this efficiently when iterating across trees.
• Compute the corresponding mutation configuration probability. Currently, there is no API in place that returns this probability for a single configuration.

[GertjanBisschop]

Using bitwise operations to keep track of the different branchtypes. To do efficient updates using the ts.edge_diffs() we should keep track of the type of each branch by storing that value in an array such that branchtype_array[edge.child] == branchtype.

The following code, initialises this array and the counts for a tree.

def init_bt_array(tree, bt_array, bt_counts):
    for node in tree.nodes(order="postorder"):
        if tree.is_sample(node):
            bt_array[node] = 1<<(node+1)
        else:
            child = tree.left_child_array[node]
            while child != tskit.NULL:
                bt_array[node] |= bt_array[child]
                child = tree.right_sib_array[child]
        bt_counts[bt_array[node]] += 1
        
>>> t = tskit.Tree.generate_balanced(4)
>>> num_nodes = t.tree_sequence.num_nodes
>>> node_branchtype_array = np.zeros(num_nodes + 1, dtype=np.uint32)    
>>> num_branchtypes = sum(2**i for i in range(t.num_samples(), 0, -1))
>>> branchtype_counts = np.zeros(num_branchtypes + 1, dtype=np.uint32)
>>> init_bt_array(t, node_branchtype_array, branchtype_counts)
>>> print(node_branchtype_array)
array([ 2,  4,  8, 16,  6, 24, 30,  0], dtype=uint32)

pinging @dreq.