only couple bgc when bgc is active

[anton-seaice]

I tested this with a Debug build, and it now runs without error :)

Closes #75

[dougiesquire]

Thanks @anton-seaice. Possibly worth also avoiding doing the accumulation when skl_bgc = .false.:

-    ionit (:,:,:)  = ionit (:,:,:) + tionit (:,:,:)*coef_ic
-    ioalg (:,:,:)  = ioalg (:,:,:) + tioalg (:,:,:)*coef_ic
+    if (skl_bgc) then
+        ionit (:,:,:)  = ionit (:,:,:) + tionit (:,:,:)*coef_ic
+        ioalg (:,:,:)  = ioalg (:,:,:) + tioalg (:,:,:)*coef_ic
+    endif

[anton-seaice]

Implemented the suggestion @dougiesquire

[dougiesquire]

Thanks @anton-seaice