Added a moisture (through epv.psi - or stratum water potential) as one of the controls on phenology,
two new stratum default parameters are added
epc.gs_psi_min  (default value -15)
epc.gs_psi_max  (defaul value -14)
These default values essentially turn off this control on leaf on/off (since rootzone psi is almost
always greater than -14 so the other controls (daylength, temperature, vpd) will dominate
To use change these values to reflect the soil moisture that should control leaf on/off