Minor updates - Aug 2014

Crop.cpp: Now simulation ends when all leaves are dead. Garbage out
possible after all leaves are deconstructed

Development.cpp: Photoperiod x temperature interaction routine revised.
"Dead" phase added.

gas_exchange.cpp: New gas exchange parameter sets added from Kim et al
(2006) and Kim et al. (2007) papers. Alternative gbw routine added but
commented out. Basically the two are identical.

nodalunit.cpp: Added new developmental phase (=appeared). Added new mass
calculation when initialized.

organ.cpp: Mass is updated based on CH2O and C_conc.

plant.cpp: total leaf mass calculation is done with respect to potential
leaf size and SLA is determined based on leaf area/leaf mass
accumulation. Other minor changes.

sheath.h: Phase2duration was renamed as growthDuration. This is also
done in leaf.cpp.