[io][grid] Add function for updating a subgrid.

What this MR does / why does DuMux need it: This MR introduces a new function within the subgrid grid manager to update an existing subgrid without changing the pointer to the subgrid. By doing so, the objects which are deduced from a subgrid (such as vtkWriter) stay consistent also after changing the structure of the subgrid.

Notes for the reviewer

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• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
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• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.