Define formula for spawn interval

[erebrus]

This should initially be based on elapsed time and number of children in play.

If you want to try to code it, you can do it in Arena._on_spawn_timer_timeout() by setting spawn_timer.wait_time (in seconds), using:

• child_count
• get_time_from_start() (in miliseconds)

[erebrus]

@AxelfTV , l added hints in the description in case you want to implement it, but otherwise just assign it with me when you have the formula and I'll do it.