Made sending out caravans in the lategame take fewer clicks

[Brilliand]

This addresses issue #167

[nroutasuo]

Great idea to set a better default value but two questions:

• Is the change to TradeConstants necessary?
• It took me awhile to understand what the value of defaultSelection is supposed to be if it's not maxSelection - maybe just breaking it down to one or two more named variables would make the code a bit easier to read?

[Brilliand]

• The change to TradeConstants is separate from the change to the default value. I noticed that after the change to Slugger Town, it's a bit unnecessary to have Mill Road Academy still accepting food for metal; and having only one "buy" resource per trade partner makes it easier to do large-scale trading in the lategame. I'm currently at a part of the game where I'm sending out water-for-metal caravans from 7 camps to fill a storage of roughly 14000, so two fewer clicks for each caravan sent feels like a big improvement. Swapping the order so water appears first in the list for Mill Road Academy would accomplish the same thing, if you prefer that.
• That's fair, I did sacrifice too much readability to keep it all in one line. I can break it down into an 'if' with multiple variables inside.

[nroutasuo]

Thanks for taking the time to clarify the code a bit. 👍

I don't think this is the right place to change what the trade partners buy, but I see why swapping the order of food/water could be convenient. Would you also change that to a swap instead of delete?