Stepper propagator definition IPP

Paul Gessinger-Befurtrequested to merge
stepper-propagator-definition-ipp into master

This replaces parts of !527 (closed).

This MR splits up the declaration and definition of methods in Propagator and EigenStepper into a separate ipp file.

Depends on !532 (merged) to be merged first.

Edited by Paul Gessinger-Befurt

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