Used in the inital game setup, returns instances of whatever is the "default" invader.
Parameters:
g - Reference to the Game object - passed to Invaders in their constructors (it gets used to do some operations on (x,y) coordinates.
Returns:
A reference to a new invader.
newInvader
public static newInvader( g)
Returns an invader of some randomly chosen specialized type. The mechanism will arrange that different invaders have different probabilities of occurrence - high valued and dangerous invaders should be less common that lower valued invaders.
Parameters:
g - Reference to the Game object, passed to invaders in their constructors and used for some operations involving (x,y) coordinates.
