[MINOR AI SUGGESTION] Pioneer's no more than cannon fodder

Version: 0.76

This is pretty minor, but Pioneers, and Pioneer type units, should be excluded from combat stacks. Perhaps they should also be treated like caravans, where they are special and I don't have to switch the size of their units every time I get a squad size upgrade, but that's a different matter.

This happened after I decided to pick a fight with the more powerful Porcupine (sorry, I always forget her name and call her that epithet under my breath). Perhaps this is due to the game being played only at Normal level on a small map and she didn't want to disband her no longer useful troops? No matter. It's minor, but thought you might like to know.