If a char valued attribute contains embedded nul characters,
then illegal xml will be generated when using the -x (ncml) flag.
There is not good solution since we have a char value '\0' that
is legal in a netcdf-c char valued attribute, but is completely
illegal in ncml (i.e. xml).
So, implemented hack is to go ahead and generate '�' entities
and then complain that we are generating illegal ncml.