Add an overridable function run after the simulation step.

[marcinofulus]

def after_step(self): x=1.123 for i in range(N): x=x+2.1*x

causes visible slow down in simulation ldc_2d, 128x128 for N~1000 For N=100 it seems to be negligible

benchmark tests of clean example:
./examples/ldc_2d.py --mode=benchmark --every=100 --max_iters=1000 --lat_nx=2048 --lat_ny=2048 ./examples/ldc_2d.py --mode=benchmark --every=100 --max_iters=1000

Before Total MLUPS: eff:822.65  comp:862.04 Total MLUPS: eff:239.87  comp:421.40

After Total MLUPS: eff:828.56  comp:863.44 Total MLUPS: eff:235.38  comp:415.61

Note that results vary by few percent from run to run.

[mjanusz]

Could you please also add the following stats to the description of the merge request:

./examples/ldc_2d.py --mode=benchmark --every=100 --max_iters=1000 --lat_nx=2048 --lat_ny=2048 ./examples/ldc_2d.py --mode=benchmark --every=100 --max_iters=1000

Both with your changes and without?