fix uninitialized residue_previous
Description
The variable residue_previous is not initialized. For production runs, this does not matter, as the variable in the uninitialized state is irrelevant, but the debug builder with -ffpe-trap flags reports this as error.
News snippet
Fix minor initialization bug.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.