PEP8 name refactoring

boundaries/boundaryconditions.py

boundaries/boundaryhandling.py

boundaries/createindexlistcython.pyx

chapman_enskog/__init__.py

chapman_enskog/chapman_enskog.py

chapman_enskog/chapman_enskog_higher_order.py

chapman_enskog/derivative.py

chapman_enskog/steady_state.py

continuous_distribution_measures.py

creationfunctions.py

cumulants.py

fieldaccess.py

forcemodels.py

geometry.py

gridvisualization.py

innerloopsplit.py

lbstep.py

macroscopic_value_kernels.py

maxwellian_equilibrium.py

methods/__init__.py