Clean submodules for data handling and finite differences

chapman_enskog/chapman_enskog.py

chapman_enskog/chapman_enskog_higher_order.py

chapman_enskog/chapman_enskog_steady_state.py

chapman_enskog/derivative.py

lbstep.py

phasefield/analytical.py

phasefield/experiments1D.py

phasefield/kerneleqs.py

phasefield/phasefieldstep.py

scenarios.py