handle zero gs1

[gregory-halverson-jpl]

BESS_JPL/canopy_energy_balance.py:70: RuntimeWarning: divide by zero encountered in divide
Ci = Ca - 1.6 * An / gs1 # [umol./mol]
BESS_JPL/canopy_energy_balance.py:76: RuntimeWarning: divide by zero encountered in divide
rs = 1.0 / (gs1 / cf * 1e-2) # [s m-1]