Add expiration_after method to FunctionOfTime

[wthrowe]

Necessary for deterministically coupling the evolution step size to the control system updates.

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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• The PR lists upgrade instructions and is labeled bugfix or
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Further comments

[wthrowe]

Squashed in the doc change.