Merge Karnov Group's excel-to-code with upstream excel_to_code

[andreasDalsgaard]

Adding support for resetting individual sheets Various changes related to named references Changes for references optimized away (zero/nil) Prevent unbounded recursion for hlookup's in same region

[tamc]

Thank you @andreasDalsgaard - this looks really helpful. I will write some tests before I merge to make sure I understand all the changes. With best wishes, Tom

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