add NS code using buffered particle fields

in the examples folder there is now "NSbufferedParticles", which is
identical to NavierStokes except for the particles.
There are probably cleaner ways of playing with these, but I need a
quick test.

examples/NSBufferedParticles.py

+import bfps
+import argparse
+import sys
+
+class NSBufferedParticles(bfps.NavierStokes):
+    """
+        Another example.
+        This class behaves identically to NavierStokes, except that it uses a
+        buffered interpolator, and the corresponding distributed_particles class.
+    """
+    standard_names = ['NSBP',
+                      'NSBP-single',
+                      'NSBP-double']
+    def launch(
+            self,
+            args = [],
+            noparticles = False,
+            **kwargs):
+        self.name = 'NSBufferedParticles-v' + bfps.__version__
+        opt = self.prepare_launch(args = args)
+        self.fill_up_fluid_code()
+        if noparticles:
+            opt.nparticles = 0
+        elif type(opt.nparticles) == int:
+            if opt.nparticles > 0:
+                self.name += '-particles'
+                self.add_3D_rFFTW_field(
+                        name = 'rFFTW_acc')
+                self.add_interpolator(
+                        name = 'cubic_spline',
+                        neighbours = opt.neighbours,
+                        smoothness = opt.smoothness,
+                        class_name = 'interpolator')
+                self.add_particles(
+                        integration_steps = [4],
+                        interpolator = 'cubic_spline',
+                        acc_name = 'rFFTW_acc',
+                        class_name = 'distributed_particles')
+        self.finalize_code()
+        self.launch_jobs(opt = opt)
+        return None
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(prog = 'NSBufferedParticles')
+    parser.add_argument(
+            '-v', '--version',
+            action = 'version',
+            version = '%(prog)s ' + bfps.__version__)
+    c = NSBufferedParticles(fluid_precision = 'single')
+    c.launch(args = sys.argv[1:])
+