Added GraphModel

[Simcon]

Generally lifted from OrigoDB:

https://raw.githubusercontent.com/DevrexLabs/OrigoDB/dev/src/OrigoDB.Core/Modeling/GraphModel.cs

Added tests including end-to-end smoke tests covering basic usage and proving journaling/replaying of graph.

Aware this is an unsolicited PR so won't feel offended if not required or fits your roadmap!

[CLAassistant]

All committers have signed the CLA.

[rofr]

This looks great, thank you. Not sure if anyone is using the graph but it's good to have at least as an example. I can merge as is or if you'd like to make the small changes mentioned in the comments, I will wait. And of course, please sign the CLA.

[Simcon]

Hi Robert. Thanks for the positive feedback! I'll resolve these changes some point this week/weekend. Looks like CLA has approved me as a different user so let me straighten that out well and we'll be good to go.

[Simcon]

I've pushed the requested changes and double-checked I've signed the CLA but some reason the status is still showing as unsigned?!

[rofr]

Odd that the CLA check is failing, let me dig into that before I merge the PR. We don't want to upset any lawyers :)

[rofr]

There's a known issue when using different email addresses for commits and the GH account itself:
cla-assistant/cla-assistant#352 (comment)

with a possible resolution proposed:
https://help.github.com/articles/why-are-my-commits-linked-to-the-wrong-user/#commits-are-not-linked-to-any-user

[Simcon]

Had to do a bit of a dance with GitHub but all seems good now.

[rofr]

Great, thanks for the contribution. If you'd like to have a go at some other bits let's have a chat on https://gitter.im/memstate/lobby