First commit.
Finished rain model.

Slight variation from original.
May have to revert after getting more data.
Nudged rain numbers a bit.
Added temperature sim.

Still requires radiation, humidity, etc.
Fixed a bug with the month counter.
Changed sim to use new stdev data.

Also slightly modified min temp formula.

Todo: Find and load skew factor for rain.
Percolation and upwards return flow is coded but untested.

Todo: lateral flow handling.
Todo: code for intercell water movement.
Vertical percolation works - tested
Nearly finished implementing soil grid.

Need to do: copy soil type classification function from ASCII sandbox.
Need to do: write function to tell color from soil type
Need to do: find Moore direction and implement lateral flow.
Still working on drawing the soilmap to the screen.
Almost ready to simulate subsurface lateral flow.

Performance issues with pixel blitting.
Finish hooking up relevant functions and fixing stuff.
Finished soil hydrology.

Todo: Erosion, surface flow, etc
Started work on plant sim. Added humidity.
Slowly filling in plantsim functions.
Added design docs for safekeeping.
Plant sim tested.

Still need to add things.

Todo: wind velocity, sun hours, sun intensity per season
Todo: plant cells and a bunch of other stuff
Tidied up Weather code
Added a weather data tuple
added functions to let a plant or other source withdraw water from the soil
added limiting via water stress