Update KH billow case to use new format and diagnostics

src/cases/kh_billow/spins.conf

+## KH billow configuration file
+
+# Spatial Parameters
+Lx = 10.54075393
+Ly = 1.0
+Lz = 1.0
+Nx = 1024
+Ny = 1
+Nz = 128
+min_x = 0
+min_y = 0
+min_z = 0
+
+# Expansion types
+type_x = FOURIER
+type_y = FREE_SLIP
+type_z = FREE_SLIP
+mapped_grid = false
+
+# Physical Parameters
+g = 9.81
+rho_0 = 1000.0
+visco = 1e-6
+kappa_rho = 1e-6
+
+# Problem Parameters (delta_rho is percentage)
+delta_u = 0.3616628264
+dz_u = 0.1
+u_loc = 0.5
+dz_rho = 0.1
+rho_loc = 0.5
+delta_rho = 0.01
+rho_perturb = 0.0e-8
+perturb_k = 2.38434
+
+# Temporal Parameters
+final_time = 200
+plot_interval = 5
+#dt_max = 0.0
+
+# Restart Options
+restart = false
+restart_time = 0.0
+restart_sequence = 0
+restart_from_dump = false
+compute_time = -1
+
+# Perturbation Parameter
+perturb = 0.5e-3
+
+# Filter Parameters
+f_cutoff = 0.6
+f_order = 2.0
+f_strength = 20.0
+
+# secondary diagnostics
+#compute_enstrophy = true
+#compute_dissipation = true
+#compute_BPE = true
+#compute_internal_to_BPE = true
+#compute_stresses_top = false
+#compute_stresses_bottom = false