lted surface using the surface tilt angle, diffuse horizontal
% irradiance, global horizontal irradiance, and sun zenith angle. Note
% that this model is not well documented and has not been published in
% any fashion (as of January 2012).
%
%% Inputs
%%
% * *|SurfTilt|* - a scalar or vector of surface tilt angles in decimal degrees. 
%     If SurfTilt is a vector it must be of the same size as all other vector
%     inputs. SurfTilt must be >=0 and <=180. The tilt angle is defined as
%     degree