add some default particles in NavierStokes

I designed NavierStokes.prepare_launch without any particle addition
mechanism, since I only care about children of NavierStokes having
particles, but then I realized that for some very simple tests I should
have basic particle functionality.

bfps/NavierStokes.py

@@ -1001,7 +1001,8 @@ class NavierStokes(_fluid_particle_base):
         return None
     def prepare_launch(
             self,
-            args = []):
+            args = [],
+            noparticles = False):
         opt = _code.prepare_launch(self, args = args)
         # with the default Lundgren forcing, I can estimate the dissipation
         # with nondefault forcing, figure out the amplitude for this viscosity
@@ -1028,6 +1029,18 @@ class NavierStokes(_fluid_particle_base):
             self.name += '-QR'
         if len(opt.src_work_dir) == 0:
             opt.src_work_dir = opt.work_dir
+        if noparticles:
+            opt.nparticles = 0
+        elif opt.nparticles > 0:
+            self.add_3D_rFFTW_field(name = 'rFFTW_acc')
+            self.add_interpolator(
+                    name = 'cubic_spline',
+                    neighbours = 1,
+                    smoothness = 1)
+            self.add_particles(
+                    integration_steps = [4],
+                    interpolator = 'cubic_spline',
+                    acc_name = 'rFFTW_acc')
         self.pars_from_namespace(opt)
         return opt
     def launch(