Ensure chebfun3/plus works fine with objects of different tech

also added a test which did not work previously

@chebfun3/plus.m

@@ -45,7 +45,16 @@
         % number of the plus operation f + g, i.e., ||f|| + ||g|| / ||f+g||
         % We first compute a rough approximate vscale for f+g:
         m = 51; % size of sampling grid
-        hVals = sample(f, m, m, m) + sample(g, m, m, m);
+
+        % trigtech and chebtech sample at different points. The else-part of the following 
+        % statement ensures that f and g are sampled at the same points even if they have different techs.
+        if (~isPeriodicTech(f) && ~isPeriodicTech(g) ) || (isPeriodicTech(f) && isPeriodicTech(g) )
+            hVals = sample(f, m, m, m) + sample(g, m, m, m);
+        else 
+            [xxf, yyf, zzf] = chebpts3(m, m, m, f.domain);
+            [xxg, yyg, zzg] = chebpts3(m, m, m, g.domain);
+            hVals = feval(f, xxf, yyf, zzf) + feval(g, xxg, yyg, zzg);
+        end
         hVscale = max(abs(hVals(:)));
         vscaleFG = [vscale(f), vscale(g)];
         kappa = sum(vscaleFG)/hVscale;

tests/chebfun3/test_plus.m

@@ -45,4 +45,11 @@
 pass(8) = rank(g) == rank(f);
 pass(9) = abs(vscale(g) - 2e100)/2e100 < 2e-2;
 
+% Does plus works fine with objects of different tech?
+ff = @(x,y,z) sin(pi*x).*cos(pi*(x+y));
+f = chebfun3(ff);
+g = chebfun3(ff,'trig');
+fmg = f - g;
+pass(10) = norm(fmg) < tol;
+
 end