Improve Microbe AI for colonies to turn on engulf mode earlier

[hhyyrylainen]

Thanks to #6698 they can now try to engulf stuff but I think the trigger condition for turning on engulf mode needs to be tweaked so that long colonies will turn on engulf mode when their front cell is close to the target.

Or at least that's my impression without reading the code and observing some multicell colonies that quite rarely turned on engulf mode when their front cell hit a small food cell.

[IdfbAn]

How can I get the front cell of the colony?

[hhyyrylainen]

Probably need to find it by checking the min distance to the prey from all cells, though that could cost too much performance and we might need some kind of shortcut to speed things up.

[IdfbAn]

What if the closest thing is to the back? Would it be fine to keep that in?

[hhyyrylainen]

The main cell is also considered a part of the colony so it is naturally a closer colony cell than the back cells.