Issue Description:

The current spawning algorithm evolves ALL pokemobs ALWAYS.

What happens:

It can get a bit boring to only see the fully evolved versions of pokemobs.

What you expected to happen:

It would be nice to see some Caterpies and Metapods, as opposed to only Butterfrees. I think this could be done without touching the level spawning. One could just make it so that there's a 10% chance evolution stops. This way you'd rarely find some level 40 Caterpies, among the Butterfrees, giving some variety to the fauna.

Steps to reproduce:

Play and see

Affected Versions (Do not use "latest"):

All