Add helper functions for users

[VEZY]

Some traversals are quite complicated to reason about. For example, users often want to traverse the nodes along an axis without visiting the branching positions. This is especially difficult when we have several scales along the axis. For example, we can visit them as follows:

descendants(axis_node, link = ["/", "<"], all = false)

What's important here is to understand what the all argument does, and not many people go read the documentation. They would rather use a function that does that instead, or at least a filter function that already exists:

descendants(axis_node, filter_fun = node -> along_axis(node, axis = "A", target = "N"))

The axis argument is used to identify the name of the nodes that are an axis, and the target argument for getting the name of the nodes we want (could be "GU" for growth units or "N" for nodes).

[VEZY]

Another example is when we want to visit the nodes along an axis (sequentially, without visiting the GU) but in a basitonic way (from tip to base). At the moment, we have to write dedicated functions, but that's a little bit difficult for entry-level users.