[libcf] c++ support


Does libcf provide a c++ API or is it just C. for example to access a file we 
can use the following line of code

   // read-only access to the file.
   NcFile dataFile("simple_xy.nc", NcFile::ReadOnly);

however lib cf require the ncid which is used in the C API 

int nccf_def_convention(int ncid);