Fixed issues with missing unlocks/dependencies/debug checks for top level cells which aren't also super cells

Not sure how we didn't notice this one before. Should've been an issue earlier. Especially since I now detected it on EAGLE_12, whereas I've been running EAGLE_12 examples for RT checks for ages.

Anyway, this should do the trick.

This fixed a handful of related bugs.

examples/EAGLE_low_z/EAGLE_12/README

@@ -26,6 +26,9 @@ o   To use GEAR-RT, configure SWIFT with
     --with-stars=basic --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-rt=GEAR_1 --with-rt-riemann-solver=GLF --with-feedback=none
 
 [technically, any other feedback scheme should work as well.]
+[the .yml file is set up to handle up to 3 photon groups, so
+ you could also be compiling with --with-rt=GEAR_3 if you want
+ to test more photon groups as well.]
 
 
 o   To use the DEBUG RT scheme, configure SWIFT with