Improve BioMech logic

[Jabbo16]

The marines usually get too far ahead and leave the tanks undefended at the backline, this usually gets the tanks killed and the marines then exposed to the enemy units.

One improvement I have thought is to split the squads by "Bio" and "Mech", setting the "Bio" squads to protect the "Mech" squads and to not get too far from them.

Also tanks wont unsiege when enemies are too close and get killed while sieged.

[Jabbo16]

6e2a906 adds better sieging/unsieging behaviour when enemies are close to the sieged tanks.

[Jabbo16]

One idea I had is adding weights when calculating the center of the squad, giving tanks more weight so the marines try to stay closer to them.

[Jabbo16]

Done with latest commits, needs a bit of tweaking yet. Can be closed as its done and works a bit better.