Additional (simple) way to compute Evapotransporation

[florianjehn]

Oudin et al (2005, Which potential evapotranspiration input for a lumped
rainfall–runoff model?) developed an simple PET calculation method. This seems to be working very good for lumped conceptual models and also needs little forcing data (Mean Temperature and Radiation).
Might be worth including.

[philippkraft]

ETpot = potential ET in L/(day m²) = mm/day
R_e(DOY, Latitude) = Extraterrestial radiation, as in Hargreaves in MJ/(day m²)
λρ = latent heat of vaporization per litre water [MJ/kg * L/kg]
Jensen-Hainse (1963): K_1 = 40, K_2 = 0
McGuiness-Bordne (1972): K_1 = 68, K_2 = 5
Oudin et al (2005): K_1 = 100, K_2 = 5

[philippkraft]

Steps to do

• Move R_e calculation to meteostation and make R_e part of Weather
• Update Hargreave - Samani to use Weather's R_e
• Implement Oudin et al ETpot with adjustable constants K_1, K_2 in ET.h