Converts a real number to a character string, using ndp to determine the number
of decimal spaces. Places the string in a field, using nfi as an indication of field width.
Appends the field to the SGS buffered string.
If ndp>0, then ndp digits are
included. If ndp<0, then only the
integer part appears. If ndp=0,
then only the integer part and the decimal point appears.