[office-formula] IMARGUMENT needs improvements

Hi members,

IMARGUMENT needs a rule for IMARGUMENT(COMPLEX(0;0)). I would expect, 
that there is a constraint X<>0+0i or that it has a note that 
IMARGUMENT(COMPLEX(0;0)) shall give an error. Or do you want to return zero?

The definition should say, that the principal value is returned. That 
means that -pi< result <= pi

kind regards
Regina