spawn people in waves

[nishoof]

• A wave can be to spawn a certain amount of people
• It spawns them all at a certain rate (such as a person every 2-3 seconds)
• Once the wave has spawned all the people it's supposed to, the next wave starts.
• The next wave can have a different rate to spawn people or number of people to spawn.
• This would make the game more fun since it could get more difficult as you go kind of.