ENH deterministic strategies for DT to RST conversion

This PR enables the implementation of different strategies for dependency to RST tree conversion.
It exposes the (previously inner) function that sorts dependents of an EDU inside out.
This function now accepts as argument a strategy.
Two baselines strategies are currently implemented: all left then all right dependents, and all right then left dependents.

At a higher level, the strategy can be specified when calling relaxed_nuclearity_from_deptree().
The default strategy is the "id" strategy, which was previously the only one implemented.
This latter strategy follows the interleaving of left and right nodes in the list of dependents of a node in the dependency tree.