White Blood Cells: Random Type Spawn

The White Blood Cells will now spawn as a random type of White Blood
Cell enemy. Each type of WBC enemy (Dumb, Smart, Smarter) is associated
with a corresponding tag (Enemy Dumb, Enemy Smart, Enemy Smarter) which
causes it to be controlled by the correct controller. The
WhiteBloodCellUnit function setType() takes in a string like "Dumb" and
adjusts the White Blood
Cell's enemy type accordingly.

If a WBC is a Dumb Enemy, for example, it will cluster only with other
WBC with the Dumb Enemy tag.

For now, the type of enemy spawned is completely random.

I also fixed the initial random fireRate of White Blood Cell units to
range between zero and their fireRateMax.

STILL NEED TO MAKE ENEMY SPAWN TYPE DEPEND ON PLAYER SCORE