Feature: node traversal from a given root.

[molpopgen]

We should support things like preorder traversal starting from a given node as (subtree) root.
The internal code already exists to support this and the relevant type simply needs a new constructor.
(The previous sentence is true for preorder traversal. Have to check others.)

Testing would involve checking that all nodes returned have a parent path through the root node and all nodes NOT returned do not have a parent path through the root.

[molpopgen]

Actually: testing is much easier as we can simply make comparisons to the output of the low-level C API calls to do the same operations.

[molpopgen]

Closed in #882