Small fixes for broken build

In the previous merge, compilation of Sorter.o went missing from the
Makefile, and a typo was introduced in the gravity_current case file;
each would prevent a build from finishing.  These are fixed.

src/Makefile

@@ -77,11 +77,11 @@ nonhydro_x: nonhydro_sw.o TArray.o T_util.o Parformer.o Splits.o Par_util.o Spli
 derek_x: derek.o TArray.o T_util.o Parformer.o ESolver.o Timestep.o Splits.o Par_util.o Split_reader.o gmres.o gmres_1d_solver.o gmres_2d_solver.o grad.o multigrid.o 
 	$(LD) $(LDFLAGS) -o $@ $^ $(LDLIBS)
 
-cases/%.x: cases/%.o cases/%.src.o TArray.o T_util.o Parformer.o ESolver.o Timestep.o NSIntegrator.o BaseCase.o \
+cases/%.x: cases/%.o cases/%.src.o TArray.o T_util.o Parformer.o ESolver.o Timestep.o NSIntegrator.o BaseCase.o Sorter.o \
            Science.o Splits.o Par_util.o Split_reader.o gmres.o gmres_1d_solver.o gmres_2d_solver.o grad.o multigrid.o Options.o timing.o
 	$(LD) $(LDFLAGS) -o $@ $^ $(LDLIBS)
 
-cases/%_x: cases/%.o cases/%.src.o TArray.o T_util.o Parformer.o ESolver.o Timestep.o NSIntegrator.o BaseCase.o \
+cases/%_x: cases/%.o cases/%.src.o TArray.o T_util.o Parformer.o ESolver.o Timestep.o NSIntegrator.o BaseCase.o Sorter.o \
            Science.o Splits.o Par_util.o Split_reader.o gmres.o gmres_1d_solver.o gmres_2d_solver.o grad.o multigrid.o Options.o timing.o
 	$(LD) $(LDFLAGS) -o $@ $^ $(LDLIBS)
 

src/cases/gravity_current/gravity_current.cpp

@@ -308,7 +308,7 @@ class userControl : public BaseCase {
                     add_diagnostic("Enst_x_tot", enst_x_tot,    header, line);
                     add_diagnostic("Max_vort_x", max_vort_x,    header, line);
                 }
-                if (Nx > 1 && Ny > 1 && compute_enstropy) {
+                if (Nx > 1 && Ny > 1 && compute_enstrophy) {
                     add_diagnostic("Enst_z_tot", enst_z_tot,    header, line);
                     add_diagnostic("Max_vort_z", max_vort_z,    header, line);
                 }