s from horizontal (e.g. surface facing up = 0, surface facing
%     horizon = 90)
% * *|DHI|* - a scalar or vector of diffuse horizontal irradiance in W/m^2. If DHI
%     is a vector it must be of the same size as all other vector inputs. 
%     DHI must be >=0.
% * *|GHI|* - a scalar or vector of global horizontal irradiance in W/m^2. If GHI
%     is a vector it must be of the same size as all other vector inputs. 
%     GHI must be >=0.
% * *|SunZen|* - a scalar or vector of apparent (refraction-c