Support for vertex and hyperedge values

[pszufe]

Support for vertex and hyperedge values following the discussion at issue #2
This also includes several new tests and a bug correction for function set_index!

[bkamins]

so maybe add a comment that trying to remove a vertex from a hyperedge when it is not present is not an error, earlier we wanted it to throw an error.

[bkamins]

I have left some comments.

A major missing part is load/save functionality (which needs a design of storage format in the first place).

Given it starts to become a more complex design maybe we should switch to TOML or JSON with storage?

[bkamins]

Also with metadata added probably we should consider saving the information about the types of a Hypergraph, as now it might start becoming relevant. This also probably means that we should implement a binary format (even based on a serialize/deserialize I think should be enough for practical purposes).

[pszufe]

According to my tests the best option for graph serialization is the BSON.jl.
On one hand BSON format provides cross-language compatibility and on the other hand this library has a good stability.

[pszufe]

On other note - since serialization is currenty being developed by the Salerno team - I definitely propose to handle that in a separate pull request.

[bkamins]

BSON.jl - OK. Serialization: who does work on serialization?

[pszufe]

I understand that @spagnuolocarmine and @aleant93 currently work on the hypergraph storage issues.

[bkamins]

This is independent (they are working on comments). We do not have do do it in this PR, but then please open an Issue to fix it later.