Expanding the relationship API

[stavrosfa]

This PR is a first pass at expanding the relationship API to make it easier to implement actual per turn deals.

I wanted to implement the Gold per turn deal, and I did, but in the process I found that I needed to alter the Relationship class to make it easier to work with. So I decided to do this first, kind of by itself.

So took this chance to also remove the boolean atWar, as I am not a huge fan of turning off and on properties to check if we are at war, so I 'd rather wrap this around a method that checks it.
Now we do this by checking if 2 civs have a relationship, but no multi turn deals, which in turn means that these 2 civs don't have a peace deal, and since they know each other, they are at war.

I have also found it quite confusing to work with stuff like p1.playerRelationships[p2].something so I wrapped what I could around a more (I think) comprehensive API that hides all the annoying things from the end user (us) and once they are set, we don't have to rack our brains everytime we use it.

That being said, it's a lot to cover, but I would love your feedback at this point (like that I overused the static keyword and microsoft will have me pay some kind of subscription to keep using it this month 😛)

I am going to try and write some unit tests for this as well, as it touches many areas, but after manually testing it with multiple scenarios, I would say most of it is ok.

Let me know what you think.