tdf#50950 sort out Calc (ISO)WEEKNUM functions

Hi Eike,

I checked the function definitions for WEEKNUM and ISOWEEKNUM, for ODFF and Excel. ODFF and Excel 
are as good as identical, except that Excel may experience problems when the date value is entered 
is text.
I intend to ignore that difference and support both WEEKNUM and ISOWEEKNUM in Calc.
As you know, the problem is that the ODFF function name ISOWEEKNUM is currently used for a function 
with WEEKNUM functionality.

I propose to put right in 2 steps:
LO version 4.5:
UI              ODFF                    ODFF-'legacy'           Excel
WEEKNUM WEEKNUM         ISOWEEKNUM                      WEEKNUM
ISOWEEKNUM      ISOWEEKNUM_TMP                                  ISOWEEKNUM
and remove UI WEEKNUM_ADD

LO version 4.6:
UI              ODFF                    ODFF-'legacy'           Excel
WEEKNUM WEEKNUM         WEEKNUM                 WEEKNUM
ISOWEEKNUM      ISOWEEKNUM              ISOWEEKNUM_TMP          ISOWEEKNUM

Any suggestions/objections etc. before I start changing the code?

Winfried