Merge branch 'bugfix/particle-file' into develop

Merging the bugfix resulted in a conflict, because I added a docstring
for the prepare_launch method in the meantime.

Conflicts:
	bfps/NavierStokes.py

bfps/NavierStokes.py

@@ -1010,7 +1010,8 @@ class NavierStokes(_fluid_particle_base):
         return None
     def prepare_launch(
             self,
-            args = []):
+            args = [],
+            noparticles = False):
         """Set up reasonable parameters.
 
         With the default Lundgren forcing applied in the band [2, 4],
@@ -1061,6 +1062,18 @@ 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(