Unify code for experiment and petri

Now a PetriDish is just an Experiment plus added graphics.

I also removed the creatures from the fridge. The genetic code has
changed enough that they're almost unrecognisable. If I want them,
I can travel back to previous commits anyway.
Refactor and rename stuff

Mainly remove references to "ponds" (that are now named "ecosystems")
and add/update package-info.java files to the packages in the Core
project.
The amplitude of the body segments' movements is genetically determined

Before this, it was hardcoded at 45. Now it can be anywhere from 1 to
80.