Review and Restructure Methods in New Flow ABC Class

[McBs]

All methods from the old Lattice class have been transferred to the new Flow ABC class. We need to review and possibly restructure these methods to ensure they are logically organized and adhere to our coding standards.

Edit by @PhiSpel:

• When you're done with this, do make sure to reactivate test_collision_fixpoint_2x_MRT

[PhiSpel]

Some, such as lattice.einsum, should be removed. See also #111

Edit: Done in #241

[PhiSpel]

lattice methods such as rho(f), u(f), entropy(f), etc. could be stored separately so that they can be accessed by tests without creating a Flow object. These methods do not require Flow-specific attributes, except sometimes the stencil weights.

• When you're done with this, do make sure to reactivate test_collision_fixpoint_2x_MRT