+ Appeared if .Integrate(..., relax=True) used after an external field was applied earlier.
  + Computation was already correct before.

- merge in work done for the TPS2paper revision:
  + Improvements (efficiency) in pure TPS2
  + TPS2AB with EE for lower order terms in first time-step

+ is done in fixpoint loop anyhow

+ already done within self._StepPrepareWithoutTaskManager()

+ it's already computed in self._StepPrepareWithoutTaskManager()
+ it's time independent

+ already assembled in self._StepPrepareWithTaskManager()