♻️ Address code duplication issues introduced in #860

[jsnel]

As also pointed out by SonarCloud in a detailed analysis this PR introduces some code duplication. We'll accept these for now, to not get in the way of the 'merge train' leading up to the introduction of the Projects feature (it could complicate rebase of subsequent PRs in the queue) but this issue will serve as a reminder to revisit this once the strain stops at the next station.

As @s-weigand puts it:
For the sake of getting on with the "merge train" I guess this is fine from my side.
Even so, I don't agree with the removing of DecayMegacomplexBase which added useless code duplication and cyclic import workarounds, just because of a disagreement about using abstract base classes

Originally posted by @s-weigand in #860 (review)

[jsnel]

To re-evaluate how much of an issues this still is, especially following changes in PR #1135 and the (expected) follow-up PR refining kinetic decay megacomplex.

[stale]

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[stale]

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