Upgrade paths (BTDRN)

For a possible upcoming game, there is some new upgrade paths.

There are 3 upgrade paths at first, but when one is at tier 3, that path splits into two, the max tier is tier 6.
 * You can't have two T3s as the same tower.
 * You can only upgrade in two paths per tower.
 * You can also only upgrade from one split path per tower.
 * As soon as you have a T5, you can't buy it again.
 * You can only have one T6 per game, unless you have some monkey knowledge that says otherwise.