Fix the default kphio values

[davidorme]

At present, the quantum yield calculations use Beni's default settings for his calibrated calculations with the soil moisture correction:

pyrealm/pyrealm/pmodel/quantum_yield.py

Line 220 in 247d35e

default_reference_kphio=0.049977,

pyrealm/pyrealm/pmodel/quantum_yield.py

Line 243 in 247d35e

default_reference_kphio=0.081785,

We need to set these to 1/8 and then update the soil moisture correction docs to explicitly say that you need to set those values to replicate the predictions of the Stocker calculations.