Implement __xludf.DUMMYFUNCTION added to sheets exported from Google Sheets

[aglim]

Is your feature request related to a problem? Please describe.
Google Sheets-specific formulas in sheets exported to Excel are wrapped with ISERROR(__xludf.DUMMYFUNCTION(original_formula), computed_value) where __xludf.DUMMYFUNCTION is a non-existent formula that always gives an error resulting in returning the computed value.

As __xludf.DUMMYFUNCTION is not implemented yet, files with Google Sheets-specific formulas can't be handled by formulas raising NotImplementedError exception.

Describe the solution you'd like
Implement __xludf.DUMMYFUNCTION which always returns an error.

[aglim]

Workaround for now is to add function in runtime with

FUNCTIONS = formulas.get_functions()
FUNCTIONS["__XLUDF.DUMMYFUNCTION"] = lambda s: formulas.NAME

[vinci1it2000]

I implemented a function as described above.