Step 6: Time Evolution (Alfvén Dynamics)

[anjor]

In krmhd/physics.py add the RHS for active fields:

1. dphi_dt computing RHS of stream function equation
2. dA_parallel_dt computing RHS of vector potential equation
3. Include viscosity/resistivity terms
4. Test: initialized Alfvén wave should propagate at correct speed

Dependencies: Step 5
Next Step: Step 7