Fix operation-counting tests with second order moment getter

lbmpy/methods/conservedquantitycomputation.py

@@ -136,8 +136,7 @@ class DensityVelocityComputation(AbstractConservedQuantityComputation):
     def equilibrium_input_equations_from_pdfs(self, pdfs):
         dim = len(self._stencil[0])
         eq_coll = get_equations_for_zeroth_and_first_order_moment(self._stencil, pdfs, self._symbolOrder0,
-                                                                  self._symbolsOrder1[:dim],
-                                                                  self._symbolsOrder2[:(dim * dim)])
+                                                                  self._symbolsOrder1[:dim])
         if self._compressible:
             eq_coll = divide_first_order_moments_by_rho(eq_coll, dim)
 
@@ -210,8 +209,7 @@ class DensityVelocityComputation(AbstractConservedQuantityComputation):
             momentum_density_output_symbols = output_quantity_names_to_symbols['momentum_density']
             mom_density_eq_coll = get_equations_for_zeroth_and_first_order_moment(self._stencil, pdfs,
                                                                                   self._symbolOrder0,
-                                                                                  self._symbolsOrder1,
-                                                                                  self._symbolsOrder2)
+                                                                                  self._symbolsOrder1)
             mom_density_eq_coll = apply_force_model_shift('macroscopic_momentum_density_shift', dim, 
                                                           mom_density_eq_coll, self._forceModel, self._compressible)
 
@@ -244,7 +242,7 @@ class DensityVelocityComputation(AbstractConservedQuantityComputation):
 
 
 def get_equations_for_zeroth_and_first_order_moment(stencil, symbolic_pdfs, symbolic_zeroth_moment,
-                                                    symbolic_first_moments, symbolic_second_moments):
+                                                    symbolic_first_moments, symbolic_second_moments=None):
     r"""
     Returns an equation system that computes the zeroth and first order moments with the least amount of operations
 
@@ -302,7 +300,8 @@ def get_equations_for_zeroth_and_first_order_moment(stencil, symbolic_pdfs, symb
     equations = []
     equations += [Assignment(symbolic_zeroth_moment, pdf_sum)]
     equations += [Assignment(u_i_sym, u_i) for u_i_sym, u_i in zip(symbolic_first_moments, u)]
-    equations += [Assignment(symbolic_second_moments[i], p[i]) for i in range(dim**2)]
+    if symbolic_second_moments:
+        equations += [Assignment(symbolic_second_moments[i], p[i]) for i in range(dim**2)]
 
     return AssignmentCollection(equations, subexpressions)