Add additional error estimates needed for variable-order stepping

[wthrowe]

Proposed changes

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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Further comments

[nilsdeppe]

LGTM, I just have a question :)

[nilsdeppe]

CI failure was in Poisson ProductOfSinusoids, so unrelated.