Update Graph to encode strictness behaviour

[swapnilraj]

Add a strict constructor to graph to encode strictness behaviour.
Implemented combinators to make it easy to migrate.

https://www.graphviz.org/doc/info/lang.html

[csicar]

Sorry for taking so long.

I'm not sure, if I already asked, but is there a reason to have Strict over just adding one edge, instead of using strict to remove the additional edges?

What does concentrate do?

[swapnilraj]

In my case, I am generating the edges instead of statically making the graph. Even though I could check in my code but I find it easier if graphviz handles it for me.

Irrespective of it, its just better for the library to have support for more graphviz attributers to encode.

Strict: This forbids the creation of multi-edges, i.e., there can be at most one edge with a given tail node and head node in the directed case.
Concentrate: merges multiedges into a single edge and causes partially parallel edges to share part of their paths

[csicar]

I see, I guess that's a good reason ;)
The changes that concentrate introduce look good 👍

If you don't mind, I would move strict to the global directive. I know graphviz does it this way, but I find it a bit confusing. Why should strict be treated different when compared to e.g. rankdir?

Basically:

DiGraph [
    global [ strict ]
    ...
]

instead of

DiGraph Strict [
   ...
]

Also: Thanks for the contribution, always happy to get new ones :)