Fixed extraction of common srt eq term

Integration of zero-centered storage has broken a simplification step central to SRT/TRT methods. This MR fixes it.

lbmpy_tests/test_srt_trt_simplifications.py

@@ -41,7 +41,7 @@ def test_simplifications_srt_d2q9_incompressible_regular():
 def test_simplifications_srt_d2q9_incompressible_zc():
     omega = sp.symbols('omega')
     method = create_srt(LBStencil(Stencil.D2Q9), omega, compressible=False,
-                        zero_centered=True, equilibrium_order=2)
+                        zero_centered=True, delta_equilibrium=True, equilibrium_order=2)
     check_method(method, [53, 46, 0], [53, 38, 0])
 
 
@@ -55,7 +55,7 @@ def test_simplifications_srt_d2q9_compressible_regular():
 def test_simplifications_srt_d2q9_compressible_zc():
     omega = sp.symbols('omega')
     method = create_srt(LBStencil(Stencil.D2Q9), omega, compressible=True,
-                        equilibrium_order=2)
+                        zero_centered=True, delta_equilibrium=True, equilibrium_order=2)
     check_method(method, [54, 58, 1], [54, 42, 1])