Fixed bug in birth calculation.
Restructuring of folder system.
Moved tests inside source dir ... possibly.
All kinds of stuff!!

In all my excitement, I forgot to commit regularly... anyway:
Moved all the remotely mathy functions out in a helper class/library.
Wrote interfaces for simulator classes.
Randomised mouse age, hooray.
Moved some simulation logic back in Field, it just made more sense.
Simulator constructor is now all good and proper.
Now all eclipsed up (four-space indentation EVERYWHERE).j
nooo eclipse, not tabs ... (NOW it's four space indentation)
Implemented range checks and exception stuff.
Various small fixes.

changed parameter order for getradius because I kept getting it wrong.
moved some of the simulation logic back to simulator, kept some in field ...
added some more sanity checks some places.
More small fixes, added more checks.
TESTS!!! super rad
VERSION 3 BUMP!!!

Added test library dependencies
Updated the README file