Implemented 2-part transmissivity v depth function.

Introduced a new parameter, DEPTH_THRESHOLD.  When water table depth is shallower than this threshold, transmissivity decays exponentially with depth as before.  When water table depth is deeper than this threshold, transmissivity decays linearly until it reaches 0.  DEPTH_THRESHOLD must be specified in the config file.
Merge pull request #3 from tbohn/feature/non-linear_transmissivity

Implemented 2-part transmissivity v depth function.