Fix COSTT error in pvl_klucher1979.m (#14)

Closes #13

Author: cwhanse

pvl_klucher1979.m

@@ -72,9 +72,14 @@
 GHI(GHI < 1E-6) = 1E-6; % Prevent division by 0 
 % Dec 2012: A bug was identified by Rob Andrews (Queens University) in this equation in PV_LIB
 % Version 1.0.  Fixed in Version 1.1.
+% COSTT is cos(angle between sun vector and array normal)
 COSTT = cosd(SurfTilt).*cosd(SunZen) + sind(SurfTilt).* ...
     sind(SunZen).*cosd(SunAz-SurfAz);
 
+% Aug 2018: Restrict COSTT>=0, so that circumsolar irradiance is not added
+% to total sky diffuse when the sun is behind the array
+COSTT(COSTT<0) = 0.0;
+
 F = 1 - ((DHI./GHI).^2);
 SkyDiffuse = DHI ...
     .* (0.5 .* (1 + cosd(SurfTilt))) ...