Introduce is_path_graph

[mtfishman]

This PR adds:

• is_path_graph for checking if a graph is a path/linear graph.
• Fixes degree, indegree, outdegree for integer vertex names.
• Introduces a Key object for unambiguous indexing of arrays, dictionaries, etc. This will help when dealing with general keys of dictionaries and vertices of NamedGraphs in nested data structures, where if the keys/vertex names are vectors it can be ambiguous whether they refer to a data structure or a key/vertex. The particular application is binary tree contraction sequences in ITensorNetworks.jl like [[1, 2], 3] which can become ambiguous if the vertex names are vectors [1, 2], [3, 4], [5, 6], i.e. [[[1, 2], [3, 4]], [5, 6]]. Using the Key wrapper that sequence can be represented as [[Key([1, 2]), Key([3, 4])], Key([5, 6])]. @JoeyT1994