Disable g[[1, 2, 3]] for getting an induced subgraph

[mtfishman]

Disable g[[1, 2, 3]] for getting an induced subgraph. The syntax is ambiguous since it isn't clear of [1, 2, 3] is a list of vertices or a single vertex.

As an alternative, we could define an iterator Vertices such that g[Vertices([1, 2, 3])] unambiguously returns and induced subgraph, with the implicit assumption that Vertices can't be used as vertex names of named graphs.