Implement ETD time-steppers
ETD methods (exponential time differencing) are great explicit methods for stiff PDEs. I have great experience with the ETD-RK4 method. We could implement these in bice/time_steppers/etd.py
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Issue: ETD methods require to split the rhs into a linear and a nonlinear part. A problem would be required to know it's linear and nonlinear parts of the rhs.