Cosine of solar zenith angle

For explanations of formulae, see http://www.itacanet.org/the-sun-as-a-source-of-energy/part-3-calculating-solar-angles/

cos_solar_zenith_angle(doy, lat, lon, dt, hr)

Arguments

doy	Day of year
lat	Latitude
lon	Longitude
dt	Timestep
hr	Hours timestep

Value

`numeric(1)` of cosine of solar zenith angle