Rename concepts.Path to concepts.HierarchicalIdentifier

concepts.Path's name clashes with java.nio.file.Path (which it already references), making it hard to work with both.

The concept as such is tied to Identifier by its documentation:

Paths also have the notion of containment, where one path is said to contain
another path if it the data set identified by the former contains all elements
of the data set represented by later.

This implies a Path is also an Identifier, which it currently is not, but a survey of its implementations shows each of them is also an Identifier.

Introduce a correct replacement for Path and remove it.