Color property change proposals implemented.

Also began work on swarmevolvers and a related demo world.
Allow :eye-color and :core-color properties proposals.
Several changes/fixes + swarmevolvers.

Added :nursery-threshold setting: the number of active pucks below
which nurseries may generate new pucks.
Widened ventbox gates in ai.world6 and ai.world7.
Vents no longer give energy to torpedoes.
Added :torpedo-energy setting.
Added rotational-difference and limit1 utility functions.
Spawned pucks retain :genome entry of :memory specified by spawning
parent.
Created swarmevolver agents and dev.world15 to demonstrate them.
Updated README in several small ways.
Version bump.