Additional (simple) way to compute Evapotransporation about cmf

[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