CCTK reference documents CCTK_Reduce as callable from Fortran

[Roland Haas]

CCTK_Reduce is a C varargs function and thus cannot be made callable from Fortran as Fortran does not have this concept of variable number of arguments (other than “otpional” which is still finite number of args).

This had been reported in 2004 already: http://cactuscode.org/pipermail/users/2004-September/001224.html along with a fix that would make the “typical” case of a single to-be-reduced grid function work as before (no code change).