Remove 'root' gate from GatingStrategy gate tree

[whitews]

Replace 'root' with None for specifying gates at the root level. This makes the gate path tuples one item shorter and eliminates the issue of using the reserved word 'root' as the name for a gate (even if that would be rare).

[whitews]

The anytree library needs a top-level Node for all nodes to be in the same tree, else we'd have multiple trees to manage. Not it is worth the trouble to remove the 'root' gate at this time, putting this issue on-hold.