Equals method for Unit

Hi!

First of all thanks for the hard work put into this game! It has been very useful for my research

I have faced the problem that if I were to look for example a Unit in a map, there is a lack of an **equals** method that helps me to find this unit. There are workarounds like using the UnitID, or by using a wrapper class over the Unit that has an **equals** method, but I think the most ideal solution is to have an **equals** method directly in the code.

Do you think this makes sense?

Start by locating the Unit class and reading how units are identified, especially the UnitID workaround described in the issue. Determine the intended equality semantics and add coverage for looking up an equivalent Unit in a map; done means the relevant lookup works without a wrapper class.

Written by the indexing model from the issue text.