Fix time conversion after rounding

[lars-reimann]

What problem do you want to solve?

Closes #712

Previously, deriving a time-converted function from a rounded function lead to the following error:

KeyError: Rounding specifications for function test_func_y are expected in the parameter dictionary 
at ['params_key_test']['rounding']['test_func_y']. These nested keys do not exist. 
If this function should not be rounded, remove the respective decorator.

This PR fixes this.

Todo

• [X] Pick an appropriate title.
• [X] Put Closes #XXXX in the first PR comment to auto-close the relevant issue once the PR is accepted. This is not applicable if there is no corresponding issue.
• [ ] ~Document PR in CHANGES.md.~ (Omitted due to small size of PR)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.49%. Comparing base (e992d7b) to head (5c5da12).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #715 +/- ## ========================================== + Coverage 89.44% 89.49% +0.05% ========================================== Files 51 51 Lines 3637 3637 ========================================== + Hits 3253 3255 +2 + Misses 384 382 -2 ```

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