Schiller force model to be used instead of Guo on non-SRT

This is supposed to be equivalent to Guo, but confirmed to be working for non-SRT. Also includes two test cases:

• one that confirms momentum conservation for all available force models
• one that checks that the moments of the force look right, but only for Schiller and Simple force models

Fixes #11 (closed)

doc/notebooks/05_tutorial_modifying_method_smagorinsky.ipynb

 ``` python
 method = create_lb_method(method='mrt', weighted=True, stencil='D2Q9', force=(1e-6, 0),
-                          relaxation_rates=[0, 0, ω, 1.9, 1.9])
+                          force_model='luo', relaxation_rates=[0, 0, ω, 1.9, 1.9])
 method