Fix bug in function parameter

Tom Fischerrequested to merge
TomFischer/ogs:FixBugInFunctionParameter into master

@ErikNixdorf discovered a design bug in using the parameter of type Function.

Also extend the Function type parameter to handle time in the expressions introducing the t variable, additionally to the currently available x, y, z.

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?
  3. Any new feature or behavior change was documented?
Edited by Tom Fischer

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