tdf#50950 sort out Calc (ISO)WEEKNUM functions

[Winfried Donkers]

Hi Eike,

> 
> So, having wrapped my head around this ...
> 
> Function-wise my proposal is
1
> * keep the Add-In WEEKNUM_ADD implementation as is, and let it get used
>   when reading/writing old BIFF Excel documents as it is done today
>   * rename in UI to WEEKNUM_EXCEL2007 or whatever seems fit to point out
>     it is a legacy function
>   * store in ODFF as COM.MICROSOFT.WEEKNUM
>   * store in OOXML as WEEKNUM
2
> * enhance the internal WEEKNUM function to actually implement what ODFF
>   defines
>   * store in ODFF as WEEKNUM
>   * store in OOXML as WEEKNUM
3
> * implement ISOWEEKNUM as defined by ODFF
>   * store in ODFF as ISOWEEKNUM
>   * store in OOXML as _xlfn.ISOWEEKNUM
> 
> For forward-compatibilty in older still supported LibreOffice branches
4
> * add functionality to map ISOWEEKNUM with one parameter to WEEKNUM with
>   two parameters, adding a second parameter with value 2 (which for
>   those implementations means week starting on Monday and first Thursday
>   is week 1 (the ISO definition)).
> 
> Does all that make sense?

Yes, it does and it sounds like a good plan.
I will start working on it, probably in two parts, first (1 and 2), followed by (3 and 4).
(My fuzzy proposal regarded 2 as a starting point.)
 
Winfried