New model to define behavior as droplets fall
Accumulate Droplets

Updated logic to accumulate volume into a single agent and to kill all
others on the patch.
Working drop dynamics

Implemented downhill4 based upon patch saturation level to distribute
water droplets.
Updated driver for random drop placement

Testing to make sure that random drop placement results in correct
water flow
Added drop behavior code

Added drop behavior code and tested droplet distribution.
Changed drop dynamics to use elevation and hardness

Rewrote setup, update, and distribute-droplets to use elevation and
hardness to control dynamics.